LithosAnanake/tools/pkcs8_ed25519.c

/* pkcs8_ed25519.c -- see pkcs8_ed25519.h. */
#define _GNU_SOURCE /* memmem() */
#include "pkcs8_ed25519.h"
#include <string.h>
#include <stdlib.h>

/*
 * `openssl genpkey` writes PEM (base64 text between BEGIN/END markers)
 * by default, not raw DER -- accept either transparently rather than
 * requiring callers to pre-convert with an external tool (consistent
 * with this file's whole point: no new host binary dependency). A small,
 * self-contained base64 decoder, since none existed anywhere in this
 * repo to reuse (checked 2026-08-26).
 *
 * Returns a malloc'd DER buffer (caller frees) and sets *der_len, or
 * NULL if this doesn't look like a well-formed PEM block. Decodes only
 * the FIRST "-----BEGIN ... -----" / "-----END ... -----" pair found;
 * good enough for a single private key, not a general PEM bundle parser.
 */
static int b64_val(uint8_t c) {
    if (c >= 'A' && c <= 'Z') return c - 'A';
    if (c >= 'a' && c <= 'z') return c - 'a' + 26;
    if (c >= '0' && c <= '9') return c - '0' + 52;
    if (c == '+') return 62;
    if (c == '/') return 63;
    return -1; /* not a base64 char (newline, '=', etc.) */
}

static uint8_t *pem_to_der(const uint8_t *pem, size_t pem_len, size_t *der_len_out) {
    const char *begin_marker = "-----BEGIN";
    const char *end_marker   = "-----END";

    const uint8_t *begin = memmem(pem, pem_len, begin_marker, strlen(begin_marker));
    if (!begin) return NULL;
    const uint8_t *line_end = memchr(begin, '\n', (size_t)(pem + pem_len - begin));
    if (!line_end) return NULL;
    const uint8_t *body_start = line_end + 1;

    const uint8_t *end = memmem(body_start, (size_t)(pem + pem_len - body_start),
                                 end_marker, strlen(end_marker));
    if (!end) return NULL;

    /* Decode base64 chars between body_start and end, skipping anything
     * that isn't a valid base64 character (newlines, stray whitespace). */
    size_t max_out = (size_t)(end - body_start); /* over-allocate, safe upper bound */
    uint8_t *out = malloc(max_out ? max_out : 1);
    if (!out) return NULL;

    size_t out_len = 0;
    int acc = 0, nbits = 0;
    for (const uint8_t *p = body_start; p < end; p++) {
        int v = b64_val(*p);
        if (v < 0) continue; /* skip newlines and '=' padding */
        acc = (acc << 6) | v;
        nbits += 6;
        if (nbits >= 8) {
            nbits -= 8;
            out[out_len++] = (uint8_t)((acc >> nbits) & 0xFF);
        }
    }

    *der_len_out = out_len;
    return out;
}

typedef struct {
    const uint8_t *p;
    size_t len;
} der_span_t;

/* Same short-form/long-form DER TLV walker as x509_ed25519.c's der_next()
 * -- see that file's comment for the full rationale. Bounds-checked
 * against limit at every step; refuses malformed input, never faults. */
static int der_next(const uint8_t **cursor, const uint8_t *limit,
                     uint8_t *tag_out, der_span_t *content_out) {
    const uint8_t *p = *cursor;
    if (p >= limit) return -1;
    uint8_t tag = *p++;

    if (p >= limit) return -1;
    uint8_t len_byte = *p++;
    size_t len;
    if (len_byte & 0x80u) {
        uint8_t nbytes = (uint8_t)(len_byte & 0x7Fu);
        if (nbytes == 0 || nbytes > 4) return -1;
        if ((size_t)(limit - p) < nbytes) return -1;
        len = 0;
        for (uint8_t i = 0; i < nbytes; i++) len = (len << 8) | *p++;
    } else {
        len = len_byte;
    }
    if ((size_t)(limit - p) < len) return -1;

    *tag_out = tag;
    content_out->p = p;
    content_out->len = len;
    *cursor = p + len;
    return 0;
}

static int pkcs8_extract_ed25519_seed_der(const uint8_t *der, size_t der_len,
                                           uint8_t seed_out[32]) {
    if (!der || !seed_out) return -1;

    const uint8_t *cur = der;
    const uint8_t *end = der + der_len;
    uint8_t tag;
    der_span_t outer;

    /* OneAsymmetricKey ::= SEQUENCE { version INTEGER, privateKeyAlgorithm
     * AlgorithmIdentifier, privateKey OCTET STRING, ... } */
    if (der_next(&cur, end, &tag, &outer) != 0 || tag != 0x30) return -1;

    const uint8_t *cur2 = outer.p;
    const uint8_t *limit2 = outer.p + outer.len;

    /* version INTEGER -- skip, don't care about the value */
    der_span_t version;
    if (der_next(&cur2, limit2, &tag, &version) != 0 || tag != 0x02) return -1;

    /* privateKeyAlgorithm ::= AlgorithmIdentifier ::= SEQUENCE { OID, params OPTIONAL } */
    der_span_t algid;
    if (der_next(&cur2, limit2, &tag, &algid) != 0 || tag != 0x30) return -1;

    const uint8_t *cur3 = algid.p;
    const uint8_t *limit3 = algid.p + algid.len;
    der_span_t oid;
    if (der_next(&cur3, limit3, &tag, &oid) != 0 || tag != 0x06) return -1;

    static const uint8_t ED25519_OID[3] = { 0x2B, 0x65, 0x70 }; /* 1.3.101.112 */
    if (oid.len != sizeof(ED25519_OID) ||
        memcmp(oid.p, ED25519_OID, sizeof(ED25519_OID)) != 0) {
        return -1;
    }

    /* privateKey ::= OCTET STRING, whose content is itself a DER-encoded
     * OCTET STRING (RFC 8410's CurvePrivateKey) wrapping the raw 32-byte
     * seed -- double-wrapped, confirmed empirically against a real
     * openssl-generated key (2026-08-26), not assumed from the RFC text
     * alone: `04 22 04 20 <32 bytes>` (outer OCTET STRING len=34,
     * containing inner OCTET STRING len=32). */
    der_span_t outer_octet;
    if (der_next(&cur2, limit2, &tag, &outer_octet) != 0 || tag != 0x04) return -1;

    const uint8_t *cur4 = outer_octet.p;
    const uint8_t *limit4 = outer_octet.p + outer_octet.len;
    der_span_t inner_octet;
    if (der_next(&cur4, limit4, &tag, &inner_octet) != 0 || tag != 0x04) return -1;
    if (inner_octet.len != 32) return -1;

    memcpy(seed_out, inner_octet.p, 32);
    return 0;
}

int pkcs8_extract_ed25519_seed(const uint8_t *data, size_t data_len,
                                uint8_t seed_out[32]) {
    if (!data || !seed_out) return -1;

    /* `openssl genpkey`'s default output is PEM, not raw DER -- accept
     * either. A PEM block always starts with "-----BEGIN" somewhere near
     * the top; anything else is assumed to already be raw DER. */
    if (memmem(data, data_len, "-----BEGIN", 10) != NULL) {
        size_t der_len = 0;
        uint8_t *der = pem_to_der(data, data_len, &der_len);
        if (!der) return -1;
        int rc = pkcs8_extract_ed25519_seed_der(der, der_len, seed_out);
        free(der);
        return rc;
    }

    return pkcs8_extract_ed25519_seed_der(data, data_len, seed_out);
}