diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 093d2e5..b646629 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -4727,6 +4727,22 @@
 
     return( PSA_ERROR_INVALID_ARGUMENT );
 }
+
+static psa_status_t psa_tls12_prf_psk_to_ms_input(
+    psa_tls12_prf_key_derivation_t *prf,
+    psa_algorithm_t hash_alg,
+    psa_key_derivation_step_t step,
+    const uint8_t *data,
+    size_t data_length )
+{
+    (void) prf;
+    (void) hash_alg;
+    (void) step;
+    (void) data;
+    (void) data_length;
+
+    return( PSA_ERROR_INVALID_ARGUMENT );
+}
 #else
 static psa_status_t psa_tls12_prf_set_seed( psa_tls12_prf_key_derivation_t *prf,
                                             const uint8_t *data,
@@ -4765,6 +4781,38 @@
     return( PSA_SUCCESS );
 }
 
+static psa_status_t psa_tls12_prf_psk_to_ms_set_key(
+    psa_tls12_prf_key_derivation_t *prf,
+    psa_algorithm_t hash_alg,
+    const uint8_t *data,
+    size_t data_length )
+{
+    psa_status_t status;
+    unsigned char pms[ 4 + 2 * PSA_ALG_TLS12_PSK_TO_MS_MAX_PSK_LEN ];
+
+    if( data_length > PSA_ALG_TLS12_PSK_TO_MS_MAX_PSK_LEN )
+        return( PSA_ERROR_INVALID_ARGUMENT );
+
+    /* Quoting RFC 4279, Section 2:
+     *
+     * The premaster secret is formed as follows: if the PSK is N octets
+     * long, concatenate a uint16 with the value N, N zero octets, a second
+     * uint16 with the value N, and the PSK itself.
+     */
+
+    pms[0] = ( data_length >> 8 ) & 0xff;
+    pms[1] = ( data_length >> 0 ) & 0xff;
+    memset( pms + 2, 0, data_length );
+    pms[2 + data_length + 0] = pms[0];
+    pms[2 + data_length + 1] = pms[1];
+    memcpy( pms + 4 + data_length, data, data_length );
+
+    status = psa_tls12_prf_set_key( prf, hash_alg, pms, 4 + 2 * data_length );
+
+    mbedtls_platform_zeroize( pms, sizeof( pms ) );
+    return( status );
+}
+
 static psa_status_t psa_tls12_prf_set_label( psa_tls12_prf_key_derivation_t *prf,
                                              const uint8_t *data,
                                              size_t data_length )
@@ -4802,6 +4850,20 @@
             return( PSA_ERROR_INVALID_ARGUMENT );
     }
 }
+
+static psa_status_t psa_tls12_prf_psk_to_ms_input(
+    psa_tls12_prf_key_derivation_t *prf,
+    psa_algorithm_t hash_alg,
+    psa_key_derivation_step_t step,
+    const uint8_t *data,
+    size_t data_length )
+{
+    if( step == PSA_KEY_DERIVATION_INPUT_SECRET )
+        return( psa_tls12_prf_psk_to_ms_set_key( prf, hash_alg,
+                                                 data, data_length ) );
+
+    return( psa_tls12_prf_input( prf, hash_alg, step, data, data_length ) );
+}
 #endif /* PSA_PRE_1_0_KEY_DERIVATION */
 #endif /* MBEDTLS_MD_C */
 
@@ -4824,15 +4886,17 @@
     else
 #endif /* MBEDTLS_MD_C */
 #if defined(MBEDTLS_MD_C)
-        /* TLS-1.2 PRF and TLS-1.2 PSK-to-MS are very similar, so share code. */
-    if( PSA_ALG_IS_TLS12_PRF( kdf_alg ) ||
-        PSA_ALG_IS_TLS12_PSK_TO_MS( kdf_alg ) )
+    if( PSA_ALG_IS_TLS12_PRF( kdf_alg ) )
     {
-        // To do: implement this
         status = psa_tls12_prf_input( &operation->ctx.tls12_prf,
                                       PSA_ALG_HKDF_GET_HASH( kdf_alg ),
                                       step, data, data_length );
-
+    }
+    else if( PSA_ALG_IS_TLS12_PSK_TO_MS( kdf_alg ) )
+    {
+        status = psa_tls12_prf_psk_to_ms_input( &operation->ctx.tls12_prf,
+                                                PSA_ALG_HKDF_GET_HASH( kdf_alg ),
+                                                step, data, data_length );
     }
     else
 #endif /* MBEDTLS_MD_C */