diff --git a/include/olm/pk.h b/include/olm/pk.h
index 3f77ef7..d41aa72 100644
--- a/include/olm/pk.h
+++ b/include/olm/pk.h
@@ -87,7 +87,7 @@ size_t olm_pk_encrypt(
     void * ciphertext, size_t ciphertext_length,
     void * mac, size_t mac_length,
     void * ephemeral_key, size_t ephemeral_key_size,
-    void * random, size_t random_length
+    const void * random, size_t random_length
 );
 
 typedef struct OlmPkDecryption OlmPkDecryption;
@@ -133,7 +133,7 @@ size_t olm_pk_generate_key_random_length(void);
 size_t olm_pk_key_from_private(
     OlmPkDecryption * decryption,
     void * pubkey, size_t pubkey_length,
-    void * privkey, size_t privkey_length
+    const void * privkey, size_t privkey_length
 );
 
 /** DEPRECATED: Use olm_pk_key_from_private
@@ -141,7 +141,7 @@ size_t olm_pk_key_from_private(
 size_t olm_pk_generate_key(
     OlmPkDecryption * decryption,
     void * pubkey, size_t pubkey_length,
-    void * privkey, size_t privkey_length
+    const void * privkey, size_t privkey_length
 );
 
 /** Returns the number of bytes needed to store a decryption object. */
@@ -230,19 +230,39 @@ size_t olm_clear_pk_signing(
 );
 
 /**
- * Initialise the signing object with a public/private keypair from a seed
+ * Initialise the signing object with a public/private keypair from a seed. The
+ * associated public key will be written to the pubkey buffer. Returns
+ * olm_error() on failure. If the public key buffer is too small then
+ * olm_pk_signing_last_error() will be "OUTPUT_BUFFER_TOO_SMALL".  If the seed
+ * buffer is too small then olm_pk_signing_last_error() will be
+ * "INPUT_BUFFER_TOO_SMALL".
  */
 size_t olm_pk_signing_key_from_seed(
     OlmPkSigning * sign,
     void * pubkey, size_t pubkey_length,
-    void * seed, size_t seed_length
+    const void * seed, size_t seed_length
 );
 
+/**
+ * The size required for the seed for initialising a signing object.
+ */
 size_t olm_pk_signing_seed_length(void);
+
+/**
+ * The size of the public key of a signing object.
+ */
 size_t olm_pk_signing_public_key_length(void);
 
-size_t olm_pk_signature_length();
+/**
+ * The size of a signature created by a signing object.
+ */
+size_t olm_pk_signature_length(void);
 
+/**
+ * Sign a message. The signature will be written to the signature
+ * buffer. Returns olm_error() on failure. If the signature buffer is too
+ * small, olm_pk_signing_last_error() will be "OUTPUT_BUFFER_TOO_SMALL".
+ */
 size_t olm_pk_sign(
     OlmPkSigning *sign,
     uint8_t const * message, size_t message_length,
diff --git a/src/pk.cpp b/src/pk.cpp
index c619aee..8d736b0 100644
--- a/src/pk.cpp
+++ b/src/pk.cpp
@@ -108,7 +108,7 @@ size_t olm_pk_encrypt(
     void * ciphertext, size_t ciphertext_length,
     void * mac, size_t mac_length,
     void * ephemeral_key, size_t ephemeral_key_size,
-    void * random, size_t random_length
+    const void * random, size_t random_length
 ) {
     if (ciphertext_length
             < olm_pk_ciphertext_length(encryption, plaintext_length)
@@ -127,7 +127,7 @@ size_t olm_pk_encrypt(
     }
 
     _olm_curve25519_key_pair ephemeral_keypair;
-    _olm_crypto_curve25519_generate_key((uint8_t *) random, &ephemeral_keypair);
+    _olm_crypto_curve25519_generate_key((const uint8_t *) random, &ephemeral_keypair);
     olm::encode_base64(
         (const uint8_t *)ephemeral_keypair.public_key.public_key,
         CURVE25519_KEY_LENGTH,
@@ -202,7 +202,7 @@ size_t olm_pk_key_length(void) {
 size_t olm_pk_key_from_private(
     OlmPkDecryption * decryption,
     void * pubkey, size_t pubkey_length,
-    void * privkey, size_t privkey_length
+    const void * privkey, size_t privkey_length
 ) {
     if (pubkey_length < olm_pk_key_length()) {
         decryption->last_error =
@@ -215,7 +215,7 @@ size_t olm_pk_key_from_private(
         return std::size_t(-1);
     }
 
-    _olm_crypto_curve25519_generate_key((uint8_t *) privkey, &decryption->key_pair);
+    _olm_crypto_curve25519_generate_key((const uint8_t *) privkey, &decryption->key_pair);
     olm::encode_base64(
         (const uint8_t *)decryption->key_pair.public_key.public_key,
         CURVE25519_KEY_LENGTH,
@@ -227,7 +227,7 @@ size_t olm_pk_key_from_private(
 size_t olm_pk_generate_key(
     OlmPkDecryption * decryption,
     void * pubkey, size_t pubkey_length,
-    void * privkey, size_t privkey_length
+    const void * privkey, size_t privkey_length
 ) {
     return olm_pk_key_from_private(decryption, pubkey, pubkey_length, privkey, privkey_length);
 }
@@ -447,7 +447,7 @@ size_t olm_pk_signing_public_key_length(void) {
 size_t olm_pk_signing_key_from_seed(
     OlmPkSigning * signing,
     void * pubkey, size_t pubkey_length,
-    void * seed, size_t seed_length
+    const void * seed, size_t seed_length
 ) {
     if (pubkey_length < olm_pk_signing_public_key_length()) {
         signing->last_error =
@@ -460,7 +460,7 @@ size_t olm_pk_signing_key_from_seed(
         return std::size_t(-1);
     }
 
-    _olm_crypto_ed25519_generate_key((uint8_t *) seed, &signing->key_pair);
+    _olm_crypto_ed25519_generate_key((const uint8_t *) seed, &signing->key_pair);
     olm::encode_base64(
         (const uint8_t *)signing->key_pair.public_key.public_key,
         ED25519_PUBLIC_KEY_LENGTH,
@@ -469,7 +469,7 @@ size_t olm_pk_signing_key_from_seed(
     return 0;
 }
 
-size_t olm_pk_signature_length() {
+size_t olm_pk_signature_length(void) {
     return olm::encode_base64_length(ED25519_SIGNATURE_LENGTH);
 }