lib25519(3)

  NAME

lib25519 - C API for the lib25519 implementation of the X25519 and Ed25519 cryptosystems

  SYNOPSIS

Using lib25519:

    #include <lib25519.h>

Link with -l25519.

X25519 key generation:

    unsigned char pk[lib25519_dh_PUBLICKEYBYTES];
    unsigned char sk[lib25519_dh_SECRETKEYBYTES];
    lib25519_dh_keypair(pk,sk);

X25519 shared-secret generation:

    unsigned char k[lib25519_dh_BYTES];
    const unsigned char pk[lib25519_dh_PUBLICKEYBYTES];
    const unsigned char sk[lib25519_dh_SECRETKEYBYTES];
    lib25519_dh(k,pk,sk);

Ed25519 key generation:

    unsigned char pk[lib25519_sign_PUBLICKEYBYTES];
    unsigned char sk[lib25519_sign_SECRETKEYBYTES];
    lib25519_sign_keypair(pk,sk);

Ed25519 signature generation:

    const unsigned char sk[lib25519_sign_SECRETKEYBYTES];
    const unsigned char m[...]; long long mlen;
    unsigned char sm[...]; long long smlen;
    lib25519_sign(sm,&smlen,m,mlen,sk);

Ed25519 signature verification and message recovery:

    const unsigned char pk[lib25519_sign_PUBLICKEYBYTES];
    const unsigned char sm[...]; long long smlen;
    unsigned char m[...]; long long mlen;
    int result;
    result = lib25519_sign_open(m,&mlen,sm,smlen,pk);

  DESCRIPTION

lib25519 is an implementation of the X25519 encryption system and the Ed25519 signature system. The stable C API for lib25519 consists of the five functions listed above.

All of these functions follow the SUPERCOP/NaCl APIs for DH (https://bench.cr.yp.to/call-dh.html) and signatures (https://bench.cr.yp.to/call-sign.html) except that

•   the function names are lib25519-specific instead of crypto_*,

•   message lengths are long long instead of unsigned long long, and

•   all functions except signature verification return void instead of int.

  X25519 KEY GENERATION

lib25519_dh_keypair(pk,sk) randomly generates Alice’s secret key sk[0], sk[1], ..., sk[lib25519_dh_SECRETKEYBYTES-1] and Alice’s corresponding public key pk[0], pk[1], ..., pk[lib25519_dh_PUBLICKEYBYTES-1].

lib25519_dh_PUBLICKEYBYTES and lib25519_dh_SECRETKEYBYTES are guaranteed to be 32, but callers wishing to allow easy substitution of other DH systems should not rely on this guarantee.

  X25519 SHARED-SECRET GENERATION

lib25519_dh(k,pk,sk) computes the X25519 secret k[0], k[1], ..., k[lib25519_dh_BYTES-1] shared between Alice and Bob, given Bob’s public key pk[0], pk[1], ..., pk[lib25519_dh_PUBLICKEYBYTES-1] and Alice’s secret key sk[0], sk[1], ..., sk[lib25519_dh_SECRETKEYBYTES-1].

lib25519_dh_PUBLICKEYBYTES, lib25519_dh_SECRETKEYBYTES, and lib25519_dh_BYTES are guaranteed to be 32, but callers wishing to allow easy substitution of other DH systems should not rely on this guarantee.

  ED25519 KEY GENERATION

lib25519_sign_keypair(pk,sk) randomly generates a secret key sk[0], sk[1], ..., sk[lib25519_sign_SECRETKEYBYTES-1] and a corresponding public key pk[0], pk[1], ..., pk[lib25519_sign_PUBLICKEYBYTES-1].

lib25519_sign_PUBLICKEYBYTES is guaranteed to be 32, and lib25519_sign_SECRETKEYBYTES is guaranteed to be 64, but callers wishing to allow easy substitution of other signature systems should not rely on these guarantees.

  ED25519 SIGNATURE GENERATION

lib25519_sign(sm,&smlen,m,mlen,sk) signs a message m[0], ..., m[mlen-1] using the signer’s secret key sk[0], sk[1], ..., sk[lib25519_sign_SECRETKEYBYTES-1], puts the length of the signed message into smlen, and puts the signed message into sm[0], sm[1], ..., sm[smlen-1].

The maximum possible length smlen is mlen+lib25519_sign_BYTES. The caller must allocate at least mlen+lib25519_sign_BYTES for sm.

lib25519_sign_SECRETKEYBYTES is guaranteed to be 64, lib25519_sign_BYTES is guaranteed to be 64, and signed messages are always exactly 64 bytes longer than messages, but callers wishing to allow easy substitution of other signature systems should not rely on these guarantees.

  ED25519 SIGNATURE VERIFICATION AND MESSAGE RECOVERY

lib25519_sign_open(m,&mlen,sm,smlen,pk) verifies the signed message in sm[0], ..., sm[smlen-1] using the signer’s public key pk[0], pk[1], ..., pk[lib25519_sign_PUBLICKEYBYTES-1]. This function puts the length of the message into mlen and puts the message into m[0], m[1], ..., m[mlen-1]. It then returns 0.

The maximum possible length mlen is smlen. The caller must allocate at least smlen bytes for m (not just some guess for the number of bytes expected in m).

If the signature fails verification, lib25519_sign_open instead returns -1. It also sets mlen to -1 and clears m[0], m[1], ..., m[smlen-1], but callers should note that other signature software does not necessarily do this; callers should always check the return value.

lib25519_sign_PUBLICKEYBYTES is guaranteed to be 32, but callers wishing to allow easy substitution of other signature systems should not rely on this guarantee.

  SEE ALSO

x25519-cli(1), ed25519-cli(1), randombytes(3)