diff --git a/tls1prf.go b/tls1prf.go
index 7043b3bc..3153fc81 100644
--- a/tls1prf.go
+++ b/tls1prf.go
@@ -16,7 +16,10 @@ func SupportsTLS1PRF() bool {
 		(vMajor >= 1 && vMinor >= 1)
 }
 
-func TLS1PRF(secret, label, seed []byte, keyLen int, h func() hash.Hash) ([]byte, error) {
+// TLS1PRF implements the TLS 1.0/1.1 pseudo-random function if h is nil,
+// else it implements the TLS 1.2 pseudo-random function.
+// The pseudo-random number will be written to result and will be of length len(result).
+func TLS1PRF(result, secret, label, seed []byte, h func() hash.Hash) error {
 	var md C.GO_EVP_MD_PTR
 	if h == nil {
 		// TLS 1.0/1.1 PRF doesn't allow to specify the hash function,
@@ -29,70 +32,73 @@ func TLS1PRF(secret, label, seed []byte, keyLen int, h func() hash.Hash) ([]byte
 		md = hashToMD(h())
 	}
 	if md == nil {
-		return nil, errors.New("unsupported hash function")
+		return errors.New("unsupported hash function")
 	}
 
 	ctx := C.go_openssl_EVP_PKEY_CTX_new_id(C.GO_EVP_PKEY_TLS1_PRF, nil)
 	if ctx == nil {
-		return nil, newOpenSSLError("EVP_PKEY_CTX_new_id")
+		return newOpenSSLError("EVP_PKEY_CTX_new_id")
 	}
 	defer func() {
 		C.go_openssl_EVP_PKEY_CTX_free(ctx)
 	}()
 
 	if C.go_openssl_EVP_PKEY_derive_init(ctx) != 1 {
-		return nil, newOpenSSLError("EVP_PKEY_derive_init")
+		return newOpenSSLError("EVP_PKEY_derive_init")
 	}
 	switch vMajor {
 	case 3:
 		if C.go_openssl_EVP_PKEY_CTX_set_tls1_prf_md(ctx, md) != 1 {
-			return nil, newOpenSSLError("EVP_PKEY_CTX_set_tls1_prf_md")
+			return newOpenSSLError("EVP_PKEY_CTX_set_tls1_prf_md")
 		}
 		if C.go_openssl_EVP_PKEY_CTX_set1_tls1_prf_secret(ctx,
 			base(secret), C.int(len(secret))) != 1 {
-			return nil, newOpenSSLError("EVP_PKEY_CTX_set1_tls1_prf_secret")
+			return newOpenSSLError("EVP_PKEY_CTX_set1_tls1_prf_secret")
 		}
 		if C.go_openssl_EVP_PKEY_CTX_add1_tls1_prf_seed(ctx,
 			base(label), C.int(len(label))) != 1 {
-			return nil, newOpenSSLError("EVP_PKEY_CTX_add1_tls1_prf_seed")
+			return newOpenSSLError("EVP_PKEY_CTX_add1_tls1_prf_seed")
 		}
 		if C.go_openssl_EVP_PKEY_CTX_add1_tls1_prf_seed(ctx,
 			base(seed), C.int(len(seed))) != 1 {
-			return nil, newOpenSSLError("EVP_PKEY_CTX_add1_tls1_prf_seed")
+			return newOpenSSLError("EVP_PKEY_CTX_add1_tls1_prf_seed")
 		}
 	case 1:
 		if C.go_openssl_EVP_PKEY_CTX_ctrl(ctx, -1,
 			C.GO1_EVP_PKEY_OP_DERIVE,
 			C.GO_EVP_PKEY_CTRL_TLS_MD,
 			0, unsafe.Pointer(md)) != 1 {
-			return nil, newOpenSSLError("EVP_PKEY_CTX_set_tls1_prf_md")
+			return newOpenSSLError("EVP_PKEY_CTX_set_tls1_prf_md")
 		}
 		if C.go_openssl_EVP_PKEY_CTX_ctrl(ctx, -1,
 			C.GO1_EVP_PKEY_OP_DERIVE,
 			C.GO_EVP_PKEY_CTRL_TLS_SECRET,
 			C.int(len(secret)), unsafe.Pointer(base(secret))) != 1 {
-			return nil, newOpenSSLError("EVP_PKEY_CTX_set1_tls1_prf_secret")
+			return newOpenSSLError("EVP_PKEY_CTX_set1_tls1_prf_secret")
 		}
 		if C.go_openssl_EVP_PKEY_CTX_ctrl(ctx, -1,
 			C.GO1_EVP_PKEY_OP_DERIVE,
 			C.GO_EVP_PKEY_CTRL_TLS_SEED,
 			C.int(len(label)), unsafe.Pointer(base(label))) != 1 {
-			return nil, newOpenSSLError("EVP_PKEY_CTX_add1_tls1_prf_seed")
+			return newOpenSSLError("EVP_PKEY_CTX_add1_tls1_prf_seed")
 		}
 		if C.go_openssl_EVP_PKEY_CTX_ctrl(ctx, -1,
 			C.GO1_EVP_PKEY_OP_DERIVE,
 			C.GO_EVP_PKEY_CTRL_TLS_SEED,
 			C.int(len(seed)), unsafe.Pointer(base(seed))) != 1 {
-			return nil, newOpenSSLError("EVP_PKEY_CTX_add1_tls1_prf_seed")
+			return newOpenSSLError("EVP_PKEY_CTX_add1_tls1_prf_seed")
 		}
 	}
-	outLen := C.size_t(keyLen)
-	out := make([]byte, outLen)
-	if C.go_openssl_EVP_PKEY_derive(ctx, base(out), &outLen) != 1 {
-		return nil, newOpenSSLError("EVP_PKEY_derive")
+	outLen := C.size_t(len(result))
+	if C.go_openssl_EVP_PKEY_derive(ctx, base(result), &outLen) != 1 {
+		return newOpenSSLError("EVP_PKEY_derive")
 	}
-	if outLen != C.size_t(keyLen) {
-		return nil, errors.New("tls1-prf: entropy limit reached")
+	// The Go standard library expects TLS1PRF to return the requested number of bytes,
+	// fail if it doesn't. While there is no known situation where this will happen,
+	// EVP_PKEY_derive handles multiple algorithms and there could be a subtle mismatch
+	// after more code changes in the future.
+	if outLen != C.size_t(len(result)) {
+		return errors.New("tls1-prf: derived less bytes than requested")
 	}
-	return out[:outLen], nil
+	return nil
 }
diff --git a/tls1prf_test.go b/tls1prf_test.go
index bc81d984..80324603 100644
--- a/tls1prf_test.go
+++ b/tls1prf_test.go
@@ -155,12 +155,13 @@ func TestTLS1PRF(t *testing.T) {
 			if !openssl.SupportsHash(tt.hash) {
 				t.Skip("skipping: hash not supported")
 			}
-			out, err := openssl.TLS1PRF(tt.secret, tt.label, tt.seed, len(tt.out), cryptoToHash(tt.hash))
+			result := make([]byte, len(tt.out))
+			err := openssl.TLS1PRF(result, tt.secret, tt.label, tt.seed, cryptoToHash(tt.hash))
 			if err != nil {
 				t.Fatalf("error deriving TLS 1.2 PRF: %v.", err)
 			}
-			if !bytes.Equal(out, tt.out) {
-				t.Errorf("incorrect key output: have %v, need %v.", out, tt.out)
+			if !bytes.Equal(result, tt.out) {
+				t.Errorf("incorrect key output: have %v, need %v.", result, tt.out)
 			}
 		})
 	}