import mbedtls ecc point (de)compression extension and use

common/mbedtls/ecc_point_compression.c

@@ -0,0 +1,130 @@
+/*
+* Not original to the mbedtls library. Taken from 
+* https://github.com/mwarning/mbedtls_ecp_compression
+* to solve mbedtls' lack of support for elliptic point
+* compression and decompression
+*
+* Released under CC0 1.0 Universal License
+*/
+
+/*
+* This is all about mbedtls_ecp_decompress() and mbedtls_ecp_compress()
+*
+* Perform X25519 / Curve25519 point compression and decompression for mbedtls.
+* As of mbedtls 2.5.1, mbedtls does not support decompression yet.
+*
+*/
+
+#define MBEDTLS_ALLOW_PRIVATE_ACCESS
+
+#include "ecc_point_compression.h"
+
+int mbedtls_ecp_decompress(
+    const mbedtls_ecp_group *grp,
+    const unsigned char *input, size_t ilen,
+    unsigned char *output, size_t *olen, size_t osize
+) {
+    int ret;
+    size_t plen;
+    mbedtls_mpi r;
+    mbedtls_mpi x;
+    mbedtls_mpi n;
+
+    plen = mbedtls_mpi_size(&grp->P);
+
+    *olen = 2 * plen + 1;
+
+    if (osize < *olen)
+        return(MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL);
+
+    if (ilen != plen + 1)
+        return(MBEDTLS_ERR_ECP_BAD_INPUT_DATA);
+
+    if (input[0] != 0x02 && input[0] != 0x03)
+        return(MBEDTLS_ERR_ECP_BAD_INPUT_DATA);
+
+    // output will consist of 0x04|X|Y
+    memcpy(output, input, ilen);
+    output[0] = 0x04;
+
+    mbedtls_mpi_init(&r);
+    mbedtls_mpi_init(&x);
+    mbedtls_mpi_init(&n);
+
+    // x <= input
+    MBEDTLS_MPI_CHK(mbedtls_mpi_read_binary(&x, input + 1, plen));
+
+    // r = x^2
+    MBEDTLS_MPI_CHK(mbedtls_mpi_mul_mpi(&r, &x, &x));
+
+    // r = x^2 + a
+    if (grp->A.p == NULL) {
+        // Special case where a is -3
+        MBEDTLS_MPI_CHK(mbedtls_mpi_sub_int(&r, &r, 3));
+    } else {
+        MBEDTLS_MPI_CHK(mbedtls_mpi_add_mpi(&r, &r, &grp->A));
+    }
+
+    // r = x^3 + ax
+    MBEDTLS_MPI_CHK(mbedtls_mpi_mul_mpi(&r, &r, &x));
+
+    // r = x^3 + ax + b
+    MBEDTLS_MPI_CHK(mbedtls_mpi_add_mpi(&r, &r, &grp->B));
+
+    // Calculate square root of r over finite field P:
+    //   r = sqrt(x^3 + ax + b) = (x^3 + ax + b) ^ ((P + 1) / 4) (mod P)
+
+    // n = P + 1
+    MBEDTLS_MPI_CHK(mbedtls_mpi_add_int(&n, &grp->P, 1));
+
+    // n = (P + 1) / 4
+    MBEDTLS_MPI_CHK(mbedtls_mpi_shift_r(&n, 2));
+
+    // r ^ ((P + 1) / 4) (mod p)
+    MBEDTLS_MPI_CHK(mbedtls_mpi_exp_mod(&r, &r, &n, &grp->P, NULL));
+
+    // Select solution that has the correct "sign" (equals odd/even solution in finite group)
+    if ((input[0] == 0x03) != mbedtls_mpi_get_bit(&r, 0)) {
+        // r = p - r
+        MBEDTLS_MPI_CHK(mbedtls_mpi_sub_mpi(&r, &grp->P, &r));
+    }
+
+    // y => output
+    ret = mbedtls_mpi_write_binary(&r, output + 1 + plen, plen);
+
+cleanup:
+    mbedtls_mpi_free(&r);
+    mbedtls_mpi_free(&x);
+    mbedtls_mpi_free(&n);
+
+    return(ret);
+}
+
+int mbedtls_ecp_compress(
+    const mbedtls_ecp_group *grp,
+    const unsigned char *input, size_t ilen,
+    unsigned char *output, size_t *olen, size_t osize
+) {
+    size_t plen;
+
+    plen = mbedtls_mpi_size(&grp->P);
+
+    *olen = plen + 1;
+
+    if (osize < *olen)
+        return(MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL);
+
+    if (ilen != 2 * plen + 1)
+        return (MBEDTLS_ERR_ECP_BAD_INPUT_DATA);
+
+    if (input[0] != 0x04)
+        return(MBEDTLS_ERR_ECP_BAD_INPUT_DATA);
+
+    // output will consist of 0x0?|X
+    memcpy(output, input, *olen);
+
+    // Encode even/odd of Y into first byte (either 0x02 or 0x03)
+    output[0] = 0x02 + (input[2 * plen] & 1);
+
+    return(0);
+}

common/mbedtls/ecc_point_compression.h

@@ -0,0 +1,32 @@
+/*
+* Not original to the mbedtls library. Taken from 
+* https://github.com/mwarning/mbedtls_ecp_compression
+* to solve mbedtls' lack of support for elliptic point
+* compression and decompression
+*
+* Released under CC0 1.0 Universal License
+*/
+
+/*
+* This is all about mbedtls_ecp_decompress() and mbedtls_ecp_compress()
+*
+* Perform X25519 / Curve25519 point compression and decompression for mbedtls.
+* As of mbedtls 2.5.1, mbedtls does not support decompression yet.
+*
+*/
+
+#include <string.h>
+
+#include "mbedtls/ecp.h"
+
+int mbedtls_ecp_decompress(
+    const mbedtls_ecp_group *grp,
+    const unsigned char *input, size_t ilen,
+    unsigned char *output, size_t *olen, size_t osize
+);
+
+int mbedtls_ecp_compress(
+    const mbedtls_ecp_group *grp,
+    const unsigned char *input, size_t ilen,
+    unsigned char *output, size_t *olen, size_t osize
+);