src of unsigned_read should be const

[mabuchner]

The ltc_math_descriptor.unsigned_read function pointer has a non-const src pointer argument, which the function is not supposed to modify

libtomcrypt/src/headers/tomcrypt_math.h

Line 179 in 673f5ce

unsigned char *src,

This is error prone as one might accidentally swap dst and src.

The src pointer should get marked as const.

int (*unsigned_read)(void *dst,
                     const unsigned char *src,
                     unsigned long  len);

[mabuchner]

In various places libtomcrypt tries to use a constant src, but it then has to cast away the constness in order to call the unsigned_read function. E.g. see this

libtomcrypt/src/pk/ecc/ecc_verify_hash.c

Line 78 in 673f5ce

if ((err = mp_read_unsigned_bin(r, (unsigned char *)sig, i)) != CRYPT_OK) { goto error; }

[levitte]

tomcrypt_math.h lacks const in quite a number of places. I supposed this is driven by tomsfastmath, which is also seriously lacking in similar cases (although interestingly enough, fp_read_unsigned_bin is properly constified, so there's really no excuse for unsigned_bin...

[levitte]

#654 should fix this particular issue, but not the broader issue of lack of const in ltc_math_descriptor. That's a more massive piece of work, also involving changes in tomsfastmath.

[levitte]

Things have evolved to become a full constification effort, in #667