http: add support for authtype and credential

[alt-git.git] / gpg-interface.c

#include "git-compat-util.h"
#include "commit.h"
#include "config.h"
#include "date.h"
#include "gettext.h"
#include "run-command.h"
#include "strbuf.h"
#include "dir.h"
#include "ident.h"
#include "gpg-interface.h"
#include "path.h"
#include "sigchain.h"
#include "tempfile.h"
#include "alias.h"

static int git_gpg_config(const char *, const char *,
                          const struct config_context *, void *);

static void gpg_interface_lazy_init(void)
{
        static int done;

        if (done)
                return;
        done = 1;
        git_config(git_gpg_config, NULL);
}

static char *configured_signing_key;
static const char *ssh_default_key_command, *ssh_allowed_signers, *ssh_revocation_file;
static enum signature_trust_level configured_min_trust_level = TRUST_UNDEFINED;

struct gpg_format {
        const char *name;
        const char *program;
        const char **verify_args;
        const char **sigs;
        int (*verify_signed_buffer)(struct signature_check *sigc,
                                    struct gpg_format *fmt,
                                    const char *signature,
                                    size_t signature_size);
        int (*sign_buffer)(struct strbuf *buffer, struct strbuf *signature,
                           const char *signing_key);
        const char *(*get_default_key)(void);
        const char *(*get_key_id)(void);
};

static const char *openpgp_verify_args[] = {
        "--keyid-format=long",
        NULL
};
static const char *openpgp_sigs[] = {
        "-----BEGIN PGP SIGNATURE-----",
        "-----BEGIN PGP MESSAGE-----",
        NULL
};

static const char *x509_verify_args[] = {
        NULL
};
static const char *x509_sigs[] = {
        "-----BEGIN SIGNED MESSAGE-----",
        NULL
};

static const char *ssh_verify_args[] = { NULL };
static const char *ssh_sigs[] = {
        "-----BEGIN SSH SIGNATURE-----",
        NULL
};

static int verify_gpg_signed_buffer(struct signature_check *sigc,
                                    struct gpg_format *fmt,
                                    const char *signature,
                                    size_t signature_size);
static int verify_ssh_signed_buffer(struct signature_check *sigc,
                                    struct gpg_format *fmt,
                                    const char *signature,
                                    size_t signature_size);
static int sign_buffer_gpg(struct strbuf *buffer, struct strbuf *signature,
                           const char *signing_key);
static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
                           const char *signing_key);

static const char *get_default_ssh_signing_key(void);

static const char *get_ssh_key_id(void);

static struct gpg_format gpg_format[] = {
        {
                .name = "openpgp",
                .program = "gpg",
                .verify_args = openpgp_verify_args,
                .sigs = openpgp_sigs,
                .verify_signed_buffer = verify_gpg_signed_buffer,
                .sign_buffer = sign_buffer_gpg,
                .get_default_key = NULL,
                .get_key_id = NULL,
        },
        {
                .name = "x509",
                .program = "gpgsm",
                .verify_args = x509_verify_args,
                .sigs = x509_sigs,
                .verify_signed_buffer = verify_gpg_signed_buffer,
                .sign_buffer = sign_buffer_gpg,
                .get_default_key = NULL,
                .get_key_id = NULL,
        },
        {
                .name = "ssh",
                .program = "ssh-keygen",
                .verify_args = ssh_verify_args,
                .sigs = ssh_sigs,
                .verify_signed_buffer = verify_ssh_signed_buffer,
                .sign_buffer = sign_buffer_ssh,
                .get_default_key = get_default_ssh_signing_key,
                .get_key_id = get_ssh_key_id,
        },
};

static struct gpg_format *use_format = &gpg_format[0];

static struct gpg_format *get_format_by_name(const char *str)
{
        int i;

        for (i = 0; i < ARRAY_SIZE(gpg_format); i++)
                if (!strcmp(gpg_format[i].name, str))
                        return gpg_format + i;
        return NULL;
}

static struct gpg_format *get_format_by_sig(const char *sig)
{
        int i, j;

        for (i = 0; i < ARRAY_SIZE(gpg_format); i++)
                for (j = 0; gpg_format[i].sigs[j]; j++)
                        if (starts_with(sig, gpg_format[i].sigs[j]))
                                return gpg_format + i;
        return NULL;
}

void signature_check_clear(struct signature_check *sigc)
{
        FREE_AND_NULL(sigc->payload);
        FREE_AND_NULL(sigc->output);
        FREE_AND_NULL(sigc->gpg_status);
        FREE_AND_NULL(sigc->signer);
        FREE_AND_NULL(sigc->key);
        FREE_AND_NULL(sigc->fingerprint);
        FREE_AND_NULL(sigc->primary_key_fingerprint);
}

/* An exclusive status -- only one of them can appear in output */
#define GPG_STATUS_EXCLUSIVE    (1<<0)
/* The status includes key identifier */
#define GPG_STATUS_KEYID        (1<<1)
/* The status includes user identifier */
#define GPG_STATUS_UID          (1<<2)
/* The status includes key fingerprints */
#define GPG_STATUS_FINGERPRINT  (1<<3)
/* The status includes trust level */
#define GPG_STATUS_TRUST_LEVEL  (1<<4)

/* Short-hand for standard exclusive *SIG status with keyid & UID */
#define GPG_STATUS_STDSIG       (GPG_STATUS_EXCLUSIVE|GPG_STATUS_KEYID|GPG_STATUS_UID)

static struct {
        char result;
        const char *check;
        unsigned int flags;
} sigcheck_gpg_status[] = {
        { 'G', "GOODSIG ", GPG_STATUS_STDSIG },
        { 'B', "BADSIG ", GPG_STATUS_STDSIG },
        { 'E', "ERRSIG ", GPG_STATUS_EXCLUSIVE|GPG_STATUS_KEYID },
        { 'X', "EXPSIG ", GPG_STATUS_STDSIG },
        { 'Y', "EXPKEYSIG ", GPG_STATUS_STDSIG },
        { 'R', "REVKEYSIG ", GPG_STATUS_STDSIG },
        { 0, "VALIDSIG ", GPG_STATUS_FINGERPRINT },
        { 0, "TRUST_", GPG_STATUS_TRUST_LEVEL },
};

/* Keep the order same as enum signature_trust_level */
static struct sigcheck_gpg_trust_level {
        const char *key;
        const char *display_key;
        enum signature_trust_level value;
} sigcheck_gpg_trust_level[] = {
        { "UNDEFINED", "undefined", TRUST_UNDEFINED },
        { "NEVER", "never", TRUST_NEVER },
        { "MARGINAL", "marginal", TRUST_MARGINAL },
        { "FULLY", "fully", TRUST_FULLY },
        { "ULTIMATE", "ultimate", TRUST_ULTIMATE },
};

static void replace_cstring(char **field, const char *line, const char *next)
{
        free(*field);

        if (line && next)
                *field = xmemdupz(line, next - line);
        else
                *field = NULL;
}

static int parse_gpg_trust_level(const char *level,
                                 enum signature_trust_level *res)
{
        size_t i;

        for (i = 0; i < ARRAY_SIZE(sigcheck_gpg_trust_level); i++) {
                if (!strcmp(sigcheck_gpg_trust_level[i].key, level)) {
                        *res = sigcheck_gpg_trust_level[i].value;
                        return 0;
                }
        }
        return 1;
}

static void parse_gpg_output(struct signature_check *sigc)
{
        const char *buf = sigc->gpg_status;
        const char *line, *next;
        int i, j;
        int seen_exclusive_status = 0;

        /* Iterate over all lines */
        for (line = buf; *line; line = strchrnul(line+1, '\n')) {
                while (*line == '\n')
                        line++;
                if (!*line)
                        break;

                /* Skip lines that don't start with GNUPG status */
                if (!skip_prefix(line, "[GNUPG:] ", &line))
                        continue;

                /* Iterate over all search strings */
                for (i = 0; i < ARRAY_SIZE(sigcheck_gpg_status); i++) {
                        if (skip_prefix(line, sigcheck_gpg_status[i].check, &line)) {
                                /*
                                 * GOODSIG, BADSIG etc. can occur only once for
                                 * each signature.  Therefore, if we had more
                                 * than one then we're dealing with multiple
                                 * signatures.  We don't support them
                                 * currently, and they're rather hard to
                                 * create, so something is likely fishy and we
                                 * should reject them altogether.
                                 */
                                if (sigcheck_gpg_status[i].flags & GPG_STATUS_EXCLUSIVE) {
                                        if (seen_exclusive_status++)
                                                goto error;
                                }

                                if (sigcheck_gpg_status[i].result)
                                        sigc->result = sigcheck_gpg_status[i].result;
                                /* Do we have key information? */
                                if (sigcheck_gpg_status[i].flags & GPG_STATUS_KEYID) {
                                        next = strchrnul(line, ' ');
                                        replace_cstring(&sigc->key, line, next);
                                        /* Do we have signer information? */
                                        if (*next && (sigcheck_gpg_status[i].flags & GPG_STATUS_UID)) {
                                                line = next + 1;
                                                next = strchrnul(line, '\n');
                                                replace_cstring(&sigc->signer, line, next);
                                        }
                                }

                                /* Do we have trust level? */
                                if (sigcheck_gpg_status[i].flags & GPG_STATUS_TRUST_LEVEL) {
                                        /*
                                         * GPG v1 and v2 differs in how the
                                         * TRUST_ lines are written.  Some
                                         * trust lines contain no additional
                                         * space-separated information for v1.
                                         */
                                        size_t trust_size = strcspn(line, " \n");
                                        char *trust = xmemdupz(line, trust_size);

                                        if (parse_gpg_trust_level(trust, &sigc->trust_level)) {
                                                free(trust);
                                                goto error;
                                        }
                                        free(trust);
                                }

                                /* Do we have fingerprint? */
                                if (sigcheck_gpg_status[i].flags & GPG_STATUS_FINGERPRINT) {
                                        const char *limit;
                                        char **field;

                                        next = strchrnul(line, ' ');
                                        replace_cstring(&sigc->fingerprint, line, next);

                                        /*
                                         * Skip interim fields.  The search is
                                         * limited to the same line since only
                                         * OpenPGP signatures has a field with
                                         * the primary fingerprint.
                                         */
                                        limit = strchrnul(line, '\n');
                                        for (j = 9; j > 0; j--) {
                                                if (!*next || limit <= next)
                                                        break;
                                                line = next + 1;
                                                next = strchrnul(line, ' ');
                                        }

                                        field = &sigc->primary_key_fingerprint;
                                        if (!j) {
                                                next = strchrnul(line, '\n');
                                                replace_cstring(field, line, next);
                                        } else {
                                                replace_cstring(field, NULL, NULL);
                                        }
                                }

                                break;
                        }
                }
        }
        return;

error:
        sigc->result = 'E';
        /* Clear partial data to avoid confusion */
        FREE_AND_NULL(sigc->primary_key_fingerprint);
        FREE_AND_NULL(sigc->fingerprint);
        FREE_AND_NULL(sigc->signer);
        FREE_AND_NULL(sigc->key);
}

static int verify_gpg_signed_buffer(struct signature_check *sigc,
                                    struct gpg_format *fmt,
                                    const char *signature,
                                    size_t signature_size)
{
        struct child_process gpg = CHILD_PROCESS_INIT;
        struct tempfile *temp;
        int ret;
        struct strbuf gpg_stdout = STRBUF_INIT;
        struct strbuf gpg_stderr = STRBUF_INIT;

        temp = mks_tempfile_t(".git_vtag_tmpXXXXXX");
        if (!temp)
                return error_errno(_("could not create temporary file"));
        if (write_in_full(temp->fd, signature, signature_size) < 0 ||
            close_tempfile_gently(temp) < 0) {
                error_errno(_("failed writing detached signature to '%s'"),
                            temp->filename.buf);
                delete_tempfile(&temp);
                return -1;
        }

        strvec_push(&gpg.args, fmt->program);
        strvec_pushv(&gpg.args, fmt->verify_args);
        strvec_pushl(&gpg.args,
                     "--status-fd=1",
                     "--verify", temp->filename.buf, "-",
                     NULL);

        sigchain_push(SIGPIPE, SIG_IGN);
        ret = pipe_command(&gpg, sigc->payload, sigc->payload_len, &gpg_stdout, 0,
                           &gpg_stderr, 0);
        sigchain_pop(SIGPIPE);

        delete_tempfile(&temp);

        ret |= !strstr(gpg_stdout.buf, "\n[GNUPG:] GOODSIG ");
        sigc->output = strbuf_detach(&gpg_stderr, NULL);
        sigc->gpg_status = strbuf_detach(&gpg_stdout, NULL);

        parse_gpg_output(sigc);

        strbuf_release(&gpg_stdout);
        strbuf_release(&gpg_stderr);

        return ret;
}

static void parse_ssh_output(struct signature_check *sigc)
{
        const char *line, *principal, *search;
        char *to_free;
        char *key = NULL;

        /*
         * ssh-keygen output should be:
         * Good "git" signature for PRINCIPAL with RSA key SHA256:FINGERPRINT
         *
         * or for valid but unknown keys:
         * Good "git" signature with RSA key SHA256:FINGERPRINT
         *
         * Note that "PRINCIPAL" can contain whitespace, "RSA" and
         * "SHA256" part could be a different token that names of
         * the algorithms used, and "FINGERPRINT" is a hexadecimal
         * string.  By finding the last occurence of " with ", we can
         * reliably parse out the PRINCIPAL.
         */
        sigc->result = 'B';
        sigc->trust_level = TRUST_NEVER;

        line = to_free = xmemdupz(sigc->output, strcspn(sigc->output, "\n"));

        if (skip_prefix(line, "Good \"git\" signature for ", &line)) {
                /* Search for the last "with" to get the full principal */
                principal = line;
                do {
                        search = strstr(line, " with ");
                        if (search)
                                line = search + 1;
                } while (search != NULL);
                if (line == principal)
                        goto cleanup;

                /* Valid signature and known principal */
                sigc->result = 'G';
                sigc->trust_level = TRUST_FULLY;
                sigc->signer = xmemdupz(principal, line - principal - 1);
        } else if (skip_prefix(line, "Good \"git\" signature with ", &line)) {
                /* Valid signature, but key unknown */
                sigc->result = 'G';
                sigc->trust_level = TRUST_UNDEFINED;
        } else {
                goto cleanup;
        }

        key = strstr(line, "key ");
        if (key) {
                sigc->fingerprint = xstrdup(strstr(line, "key ") + 4);
                sigc->key = xstrdup(sigc->fingerprint);
        } else {
                /*
                 * Output did not match what we expected
                 * Treat the signature as bad
                 */
                sigc->result = 'B';
        }

cleanup:
        free(to_free);
}

static int verify_ssh_signed_buffer(struct signature_check *sigc,
                                    struct gpg_format *fmt,
                                    const char *signature,
                                    size_t signature_size)
{
        struct child_process ssh_keygen = CHILD_PROCESS_INIT;
        struct tempfile *buffer_file;
        int ret = -1;
        const char *line;
        char *principal;
        struct strbuf ssh_principals_out = STRBUF_INIT;
        struct strbuf ssh_principals_err = STRBUF_INIT;
        struct strbuf ssh_keygen_out = STRBUF_INIT;
        struct strbuf ssh_keygen_err = STRBUF_INIT;
        struct strbuf verify_time = STRBUF_INIT;
        const struct date_mode verify_date_mode = {
                .type = DATE_STRFTIME,
                .strftime_fmt = "%Y%m%d%H%M%S",
                /* SSH signing key validity has no timezone information - Use the local timezone */
                .local = 1,
        };

        if (!ssh_allowed_signers) {
                error(_("gpg.ssh.allowedSignersFile needs to be configured and exist for ssh signature verification"));
                return -1;
        }

        buffer_file = mks_tempfile_t(".git_vtag_tmpXXXXXX");
        if (!buffer_file)
                return error_errno(_("could not create temporary file"));
        if (write_in_full(buffer_file->fd, signature, signature_size) < 0 ||
            close_tempfile_gently(buffer_file) < 0) {
                error_errno(_("failed writing detached signature to '%s'"),
                            buffer_file->filename.buf);
                delete_tempfile(&buffer_file);
                return -1;
        }

        if (sigc->payload_timestamp)
                strbuf_addf(&verify_time, "-Overify-time=%s",
                        show_date(sigc->payload_timestamp, 0, &verify_date_mode));

        /* Find the principal from the signers */
        strvec_pushl(&ssh_keygen.args, fmt->program,
                     "-Y", "find-principals",
                     "-f", ssh_allowed_signers,
                     "-s", buffer_file->filename.buf,
                     verify_time.buf,
                     NULL);
        ret = pipe_command(&ssh_keygen, NULL, 0, &ssh_principals_out, 0,
                           &ssh_principals_err, 0);
        if (ret && strstr(ssh_principals_err.buf, "usage:")) {
                error(_("ssh-keygen -Y find-principals/verify is needed for ssh signature verification (available in openssh version 8.2p1+)"));
                goto out;
        }
        if (ret || !ssh_principals_out.len) {
                /*
                 * We did not find a matching principal in the allowedSigners
                 * Check without validation
                 */
                child_process_init(&ssh_keygen);
                strvec_pushl(&ssh_keygen.args, fmt->program,
                             "-Y", "check-novalidate",
                             "-n", "git",
                             "-s", buffer_file->filename.buf,
                             verify_time.buf,
                             NULL);
                pipe_command(&ssh_keygen, sigc->payload, sigc->payload_len,
                                   &ssh_keygen_out, 0, &ssh_keygen_err, 0);

                /*
                 * Fail on unknown keys
                 * we still call check-novalidate to display the signature info
                 */
                ret = -1;
        } else {
                /* Check every principal we found (one per line) */
                const char *next;
                for (line = ssh_principals_out.buf;
                     *line;
                     line = next) {
                        const char *end_of_text;

                        next = end_of_text = strchrnul(line, '\n');

                         /* Did we find a LF, and did we have CR before it? */
                        if (*end_of_text &&
                            line < end_of_text &&
                            end_of_text[-1] == '\r')
                                end_of_text--;

                        /* Unless we hit NUL, skip over the LF we found */
                        if (*next)
                                next++;

                        /* Not all lines are data.  Skip empty ones */
                        if (line == end_of_text)
                                continue;

                        /* We now know we have an non-empty line. Process it */
                        principal = xmemdupz(line, end_of_text - line);

                        child_process_init(&ssh_keygen);
                        strbuf_release(&ssh_keygen_out);
                        strbuf_release(&ssh_keygen_err);
                        strvec_push(&ssh_keygen.args, fmt->program);
                        /*
                         * We found principals
                         * Try with each until we find a match
                         */
                        strvec_pushl(&ssh_keygen.args, "-Y", "verify",
                                     "-n", "git",
                                     "-f", ssh_allowed_signers,
                                     "-I", principal,
                                     "-s", buffer_file->filename.buf,
                                     verify_time.buf,
                                     NULL);

                        if (ssh_revocation_file) {
                                if (file_exists(ssh_revocation_file)) {
                                        strvec_pushl(&ssh_keygen.args, "-r",
                                                     ssh_revocation_file, NULL);
                                } else {
                                        warning(_("ssh signing revocation file configured but not found: %s"),
                                                ssh_revocation_file);
                                }
                        }

                        sigchain_push(SIGPIPE, SIG_IGN);
                        ret = pipe_command(&ssh_keygen, sigc->payload, sigc->payload_len,
                                           &ssh_keygen_out, 0, &ssh_keygen_err, 0);
                        sigchain_pop(SIGPIPE);

                        FREE_AND_NULL(principal);

                        if (!ret)
                                ret = !starts_with(ssh_keygen_out.buf, "Good");

                        if (!ret)
                                break;
                }
        }

        strbuf_stripspace(&ssh_keygen_out, '\0');
        strbuf_stripspace(&ssh_keygen_err, '\0');
        /* Add stderr outputs to show the user actual ssh-keygen errors */
        strbuf_add(&ssh_keygen_out, ssh_principals_err.buf, ssh_principals_err.len);
        strbuf_add(&ssh_keygen_out, ssh_keygen_err.buf, ssh_keygen_err.len);
        sigc->output = strbuf_detach(&ssh_keygen_out, NULL);
        sigc->gpg_status = xstrdup(sigc->output);

        parse_ssh_output(sigc);

out:
        if (buffer_file)
                delete_tempfile(&buffer_file);
        strbuf_release(&ssh_principals_out);
        strbuf_release(&ssh_principals_err);
        strbuf_release(&ssh_keygen_out);
        strbuf_release(&ssh_keygen_err);
        strbuf_release(&verify_time);

        return ret;
}

static int parse_payload_metadata(struct signature_check *sigc)
{
        const char *ident_line = NULL;
        size_t ident_len;
        struct ident_split ident;
        const char *signer_header;

        switch (sigc->payload_type) {
        case SIGNATURE_PAYLOAD_COMMIT:
                signer_header = "committer";
                break;
        case SIGNATURE_PAYLOAD_TAG:
                signer_header = "tagger";
                break;
        case SIGNATURE_PAYLOAD_UNDEFINED:
        case SIGNATURE_PAYLOAD_PUSH_CERT:
                /* Ignore payloads we don't want to parse */
                return 0;
        default:
                BUG("invalid value for sigc->payload_type");
        }

        ident_line = find_commit_header(sigc->payload, signer_header, &ident_len);
        if (!ident_line || !ident_len)
                return 1;

        if (split_ident_line(&ident, ident_line, ident_len))
                return 1;

        if (!sigc->payload_timestamp && ident.date_begin && ident.date_end)
                sigc->payload_timestamp = parse_timestamp(ident.date_begin, NULL, 10);

        return 0;
}

int check_signature(struct signature_check *sigc,
                    const char *signature, size_t slen)
{
        struct gpg_format *fmt;
        int status;

        gpg_interface_lazy_init();

        sigc->result = 'N';
        sigc->trust_level = TRUST_UNDEFINED;

        fmt = get_format_by_sig(signature);
        if (!fmt)
                die(_("bad/incompatible signature '%s'"), signature);

        if (parse_payload_metadata(sigc))
                return 1;

        status = fmt->verify_signed_buffer(sigc, fmt, signature, slen);

        if (status && !sigc->output)
                return !!status;

        status |= sigc->result != 'G';
        status |= sigc->trust_level < configured_min_trust_level;

        return !!status;
}

void print_signature_buffer(const struct signature_check *sigc, unsigned flags)
{
        const char *output = flags & GPG_VERIFY_RAW ? sigc->gpg_status :
                                                            sigc->output;

        if (flags & GPG_VERIFY_VERBOSE && sigc->payload)
                fwrite(sigc->payload, 1, sigc->payload_len, stdout);

        if (output)
                fputs(output, stderr);
}

size_t parse_signed_buffer(const char *buf, size_t size)
{
        size_t len = 0;
        size_t match = size;
        while (len < size) {
                const char *eol;

                if (get_format_by_sig(buf + len))
                        match = len;

                eol = memchr(buf + len, '\n', size - len);
                len += eol ? eol - (buf + len) + 1 : size - len;
        }
        return match;
}

int parse_signature(const char *buf, size_t size, struct strbuf *payload, struct strbuf *signature)
{
        size_t match = parse_signed_buffer(buf, size);
        if (match != size) {
                strbuf_add(payload, buf, match);
                remove_signature(payload);
                strbuf_add(signature, buf + match, size - match);
                return 1;
        }
        return 0;
}

void set_signing_key(const char *key)
{
        gpg_interface_lazy_init();

        free(configured_signing_key);
        configured_signing_key = xstrdup(key);
}

static int git_gpg_config(const char *var, const char *value,
                          const struct config_context *ctx UNUSED,
                          void *cb UNUSED)
{
        struct gpg_format *fmt = NULL;
        char *fmtname = NULL;
        char *trust;
        int ret;

        if (!strcmp(var, "user.signingkey")) {
                if (!value)
                        return config_error_nonbool(var);
                set_signing_key(value);
                return 0;
        }

        if (!strcmp(var, "gpg.format")) {
                if (!value)
                        return config_error_nonbool(var);
                fmt = get_format_by_name(value);
                if (!fmt)
                        return error(_("invalid value for '%s': '%s'"),
                                     var, value);
                use_format = fmt;
                return 0;
        }

        if (!strcmp(var, "gpg.mintrustlevel")) {
                if (!value)
                        return config_error_nonbool(var);

                trust = xstrdup_toupper(value);
                ret = parse_gpg_trust_level(trust, &configured_min_trust_level);
                free(trust);

                if (ret)
                        return error(_("invalid value for '%s': '%s'"),
                                     var, value);
                return 0;
        }

        if (!strcmp(var, "gpg.ssh.defaultkeycommand"))
                return git_config_string(&ssh_default_key_command, var, value);

        if (!strcmp(var, "gpg.ssh.allowedsignersfile"))
                return git_config_pathname(&ssh_allowed_signers, var, value);

        if (!strcmp(var, "gpg.ssh.revocationfile"))
                return git_config_pathname(&ssh_revocation_file, var, value);

        if (!strcmp(var, "gpg.program") || !strcmp(var, "gpg.openpgp.program"))
                fmtname = "openpgp";

        if (!strcmp(var, "gpg.x509.program"))
                fmtname = "x509";

        if (!strcmp(var, "gpg.ssh.program"))
                fmtname = "ssh";

        if (fmtname) {
                fmt = get_format_by_name(fmtname);
                return git_config_string(&fmt->program, var, value);
        }

        return 0;
}

/*
 * Returns 1 if `string` contains a literal ssh key, 0 otherwise
 * `key` will be set to the start of the actual key if a prefix is present.
 */
static int is_literal_ssh_key(const char *string, const char **key)
{
        if (skip_prefix(string, "key::", key))
                return 1;
        if (starts_with(string, "ssh-")) {
                *key = string;
                return 1;
        }
        return 0;
}

static char *get_ssh_key_fingerprint(const char *signing_key)
{
        struct child_process ssh_keygen = CHILD_PROCESS_INIT;
        int ret = -1;
        struct strbuf fingerprint_stdout = STRBUF_INIT;
        struct strbuf **fingerprint;
        char *fingerprint_ret;
        const char *literal_key = NULL;

        /*
         * With SSH Signing this can contain a filename or a public key
         * For textual representation we usually want a fingerprint
         */
        if (is_literal_ssh_key(signing_key, &literal_key)) {
                strvec_pushl(&ssh_keygen.args, "ssh-keygen", "-lf", "-", NULL);
                ret = pipe_command(&ssh_keygen, literal_key,
                                   strlen(literal_key), &fingerprint_stdout, 0,
                                   NULL, 0);
        } else {
                strvec_pushl(&ssh_keygen.args, "ssh-keygen", "-lf",
                             configured_signing_key, NULL);
                ret = pipe_command(&ssh_keygen, NULL, 0, &fingerprint_stdout, 0,
                                   NULL, 0);
        }

        if (!!ret)
                die_errno(_("failed to get the ssh fingerprint for key '%s'"),
                          signing_key);

        fingerprint = strbuf_split_max(&fingerprint_stdout, ' ', 3);
        if (!fingerprint[1])
                die_errno(_("failed to get the ssh fingerprint for key '%s'"),
                          signing_key);

        fingerprint_ret = strbuf_detach(fingerprint[1], NULL);
        strbuf_list_free(fingerprint);
        strbuf_release(&fingerprint_stdout);
        return fingerprint_ret;
}

/* Returns the first public key from an ssh-agent to use for signing */
static const char *get_default_ssh_signing_key(void)
{
        struct child_process ssh_default_key = CHILD_PROCESS_INIT;
        int ret = -1;
        struct strbuf key_stdout = STRBUF_INIT, key_stderr = STRBUF_INIT;
        struct strbuf **keys;
        char *key_command = NULL;
        const char **argv;
        int n;
        char *default_key = NULL;
        const char *literal_key = NULL;

        if (!ssh_default_key_command)
                die(_("either user.signingkey or gpg.ssh.defaultKeyCommand needs to be configured"));

        key_command = xstrdup(ssh_default_key_command);
        n = split_cmdline(key_command, &argv);

        if (n < 0)
                die("malformed build-time gpg.ssh.defaultKeyCommand: %s",
                    split_cmdline_strerror(n));

        strvec_pushv(&ssh_default_key.args, argv);
        ret = pipe_command(&ssh_default_key, NULL, 0, &key_stdout, 0,
                           &key_stderr, 0);

        if (!ret) {
                keys = strbuf_split_max(&key_stdout, '\n', 2);
                if (keys[0] && is_literal_ssh_key(keys[0]->buf, &literal_key)) {
                        /*
                         * We only use `is_literal_ssh_key` here to check validity
                         * The prefix will be stripped when the key is used.
                         */
                        default_key = strbuf_detach(keys[0], NULL);
                } else {
                        warning(_("gpg.ssh.defaultKeyCommand succeeded but returned no keys: %s %s"),
                                key_stderr.buf, key_stdout.buf);
                }

                strbuf_list_free(keys);
        } else {
                warning(_("gpg.ssh.defaultKeyCommand failed: %s %s"),
                        key_stderr.buf, key_stdout.buf);
        }

        free(key_command);
        free(argv);
        strbuf_release(&key_stdout);

        return default_key;
}

static const char *get_ssh_key_id(void) {
        return get_ssh_key_fingerprint(get_signing_key());
}

/* Returns a textual but unique representation of the signing key */
const char *get_signing_key_id(void)
{
        gpg_interface_lazy_init();

        if (use_format->get_key_id) {
                return use_format->get_key_id();
        }

        /* GPG/GPGSM only store a key id on this variable */
        return get_signing_key();
}

const char *get_signing_key(void)
{
        gpg_interface_lazy_init();

        if (configured_signing_key)
                return configured_signing_key;
        if (use_format->get_default_key) {
                return use_format->get_default_key();
        }

        return git_committer_info(IDENT_STRICT | IDENT_NO_DATE);
}

const char *gpg_trust_level_to_str(enum signature_trust_level level)
{
        struct sigcheck_gpg_trust_level *trust;

        if (level < 0 || level >= ARRAY_SIZE(sigcheck_gpg_trust_level))
                BUG("invalid trust level requested %d", level);

        trust = &sigcheck_gpg_trust_level[level];
        if (trust->value != level)
                BUG("sigcheck_gpg_trust_level[] unsorted");

        return sigcheck_gpg_trust_level[level].display_key;
}

int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *signing_key)
{
        gpg_interface_lazy_init();

        return use_format->sign_buffer(buffer, signature, signing_key);
}

/*
 * Strip CR from the line endings, in case we are on Windows.
 * NEEDSWORK: make it trim only CRs before LFs and rename
 */
static void remove_cr_after(struct strbuf *buffer, size_t offset)
{
        size_t i, j;

        for (i = j = offset; i < buffer->len; i++) {
                if (buffer->buf[i] != '\r') {
                        if (i != j)
                                buffer->buf[j] = buffer->buf[i];
                        j++;
                }
        }
        strbuf_setlen(buffer, j);
}

static int sign_buffer_gpg(struct strbuf *buffer, struct strbuf *signature,
                          const char *signing_key)
{
        struct child_process gpg = CHILD_PROCESS_INIT;
        int ret;
        size_t bottom;
        const char *cp;
        struct strbuf gpg_status = STRBUF_INIT;

        strvec_pushl(&gpg.args,
                     use_format->program,
                     "--status-fd=2",
                     "-bsau", signing_key,
                     NULL);

        bottom = signature->len;

        /*
         * When the username signingkey is bad, program could be terminated
         * because gpg exits without reading and then write gets SIGPIPE.
         */
        sigchain_push(SIGPIPE, SIG_IGN);
        ret = pipe_command(&gpg, buffer->buf, buffer->len,
                           signature, 1024, &gpg_status, 0);
        sigchain_pop(SIGPIPE);

        for (cp = gpg_status.buf;
             cp && (cp = strstr(cp, "[GNUPG:] SIG_CREATED "));
             cp++) {
                if (cp == gpg_status.buf || cp[-1] == '\n')
                        break; /* found */
        }
        ret |= !cp;
        if (ret) {
                error(_("gpg failed to sign the data:\n%s"),
                      gpg_status.len ? gpg_status.buf : "(no gpg output)");
                strbuf_release(&gpg_status);
                return -1;
        }
        strbuf_release(&gpg_status);

        /* Strip CR from the line endings, in case we are on Windows. */
        remove_cr_after(signature, bottom);

        return 0;
}

static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
                           const char *signing_key)
{
        struct child_process signer = CHILD_PROCESS_INIT;
        int ret = -1;
        size_t bottom, keylen;
        struct strbuf signer_stderr = STRBUF_INIT;
        struct tempfile *key_file = NULL, *buffer_file = NULL;
        char *ssh_signing_key_file = NULL;
        struct strbuf ssh_signature_filename = STRBUF_INIT;
        const char *literal_key = NULL;
        int literal_ssh_key = 0;

        if (!signing_key || signing_key[0] == '\0')
                return error(
                        _("user.signingKey needs to be set for ssh signing"));

        if (is_literal_ssh_key(signing_key, &literal_key)) {
                /* A literal ssh key */
                literal_ssh_key = 1;
                key_file = mks_tempfile_t(".git_signing_key_tmpXXXXXX");
                if (!key_file)
                        return error_errno(
                                _("could not create temporary file"));
                keylen = strlen(literal_key);
                if (write_in_full(key_file->fd, literal_key, keylen) < 0 ||
                    close_tempfile_gently(key_file) < 0) {
                        error_errno(_("failed writing ssh signing key to '%s'"),
                                    key_file->filename.buf);
                        goto out;
                }
                ssh_signing_key_file = strbuf_detach(&key_file->filename, NULL);
        } else {
                /* We assume a file */
                ssh_signing_key_file = interpolate_path(signing_key, 1);
        }

        buffer_file = mks_tempfile_t(".git_signing_buffer_tmpXXXXXX");
        if (!buffer_file) {
                error_errno(_("could not create temporary file"));
                goto out;
        }

        if (write_in_full(buffer_file->fd, buffer->buf, buffer->len) < 0 ||
            close_tempfile_gently(buffer_file) < 0) {
                error_errno(_("failed writing ssh signing key buffer to '%s'"),
                            buffer_file->filename.buf);
                goto out;
        }

        strvec_pushl(&signer.args, use_format->program,
                     "-Y", "sign",
                     "-n", "git",
                     "-f", ssh_signing_key_file,
                     NULL);
        if (literal_ssh_key)
                strvec_push(&signer.args, "-U");
        strvec_push(&signer.args, buffer_file->filename.buf);

        sigchain_push(SIGPIPE, SIG_IGN);
        ret = pipe_command(&signer, NULL, 0, NULL, 0, &signer_stderr, 0);
        sigchain_pop(SIGPIPE);

        if (ret) {
                if (strstr(signer_stderr.buf, "usage:"))
                        error(_("ssh-keygen -Y sign is needed for ssh signing (available in openssh version 8.2p1+)"));

                ret = error("%s", signer_stderr.buf);
                goto out;
        }

        bottom = signature->len;

        strbuf_addbuf(&ssh_signature_filename, &buffer_file->filename);
        strbuf_addstr(&ssh_signature_filename, ".sig");
        if (strbuf_read_file(signature, ssh_signature_filename.buf, 0) < 0) {
                ret = error_errno(
                        _("failed reading ssh signing data buffer from '%s'"),
                        ssh_signature_filename.buf);
                goto out;
        }
        /* Strip CR from the line endings, in case we are on Windows. */
        remove_cr_after(signature, bottom);

out:
        if (key_file)
                delete_tempfile(&key_file);
        if (buffer_file)
                delete_tempfile(&buffer_file);
        if (ssh_signature_filename.len)
                unlink_or_warn(ssh_signature_filename.buf);
        strbuf_release(&signer_stderr);
        strbuf_release(&ssh_signature_filename);
        FREE_AND_NULL(ssh_signing_key_file);
        return ret;
}