diff --git a/lib/std/crypto/tls/Client.zig b/lib/std/crypto/tls/Client.zig
index f9c897e3f0..8cfa942399 100644
--- a/lib/std/crypto/tls/Client.zig
+++ b/lib/std/crypto/tls/Client.zig
@@ -320,6 +320,7 @@ pub fn init(input: *Reader, output: *Writer, options: Options) InitError!Client
     var handshake_state: HandshakeState = .hello;
     var handshake_cipher: tls.HandshakeCipher = undefined;
     var main_cert_pub_key: CertificatePublicKey = undefined;
+    var tls12_negotiated_group: ?tls.NamedGroup = null;
     const now_sec = std.time.timestamp();
 
     var cleartext_fragment_start: usize = 0;
@@ -679,6 +680,7 @@ pub fn init(input: *Reader, output: *Writer, options: Options) InitError!Client
                         const curve_type = hsd.decode(u8);
                         if (curve_type != 0x03) return error.TlsIllegalParameter; // named_curve
                         const named_group = hsd.decode(tls.NamedGroup);
+                        tls12_negotiated_group = named_group;
                         const key_size = hsd.decode(u8);
                         try hsd.ensure(key_size);
                         const server_pub_key = hsd.slice(key_size);
@@ -691,10 +693,19 @@ pub fn init(input: *Reader, output: *Writer, options: Options) InitError!Client
                         if (cipher_state != .cleartext) return error.TlsUnexpectedMessage;
                         if (handshake_state != .server_hello_done) return error.TlsUnexpectedMessage;
 
-                        const client_key_exchange_msg = .{@intFromEnum(tls.ContentType.handshake)} ++
+                        const public_key_bytes: []const u8 = switch (tls12_negotiated_group orelse .secp256r1) {
+                            .secp256r1 => &key_share.secp256r1_kp.public_key.toUncompressedSec1(),
+                            .secp384r1 => &key_share.secp384r1_kp.public_key.toUncompressedSec1(),
+                            .x25519 => &key_share.x25519_kp.public_key,
+                            else => return error.TlsIllegalParameter,
+                        };
+
+                        const client_key_exchange_prefix = .{@intFromEnum(tls.ContentType.handshake)} ++
                             int(u16, @intFromEnum(tls.ProtocolVersion.tls_1_2)) ++
-                            array(u16, u8, .{@intFromEnum(tls.HandshakeType.client_key_exchange)} ++
-                                array(u24, u8, array(u8, u8, key_share.secp256r1_kp.public_key.toUncompressedSec1())));
+                            int(u16, @intCast(public_key_bytes.len + 5)) ++ // record length
+                            .{@intFromEnum(tls.HandshakeType.client_key_exchange)} ++
+                            int(u24, @intCast(public_key_bytes.len + 1)) ++ // handshake message length
+                            .{@as(u8, @intCast(public_key_bytes.len))}; // public key length
                         const client_change_cipher_spec_msg = .{@intFromEnum(tls.ContentType.change_cipher_spec)} ++
                             int(u16, @intFromEnum(tls.ProtocolVersion.tls_1_2)) ++
                             array(u16, tls.ChangeCipherSpecType, .{.change_cipher_spec});
@@ -703,7 +714,8 @@ pub fn init(input: *Reader, output: *Writer, options: Options) InitError!Client
                             inline else => |*p| {
                                 const P = @TypeOf(p.*).A;
                                 p.transcript_hash.update(wrapped_handshake);
-                                p.transcript_hash.update(client_key_exchange_msg[tls.record_header_len..]);
+                                p.transcript_hash.update(client_key_exchange_prefix[tls.record_header_len..]);
+                                p.transcript_hash.update(public_key_bytes);
                                 const master_secret = hmacExpandLabel(P.Hmac, pre_master_secret, &.{
                                     "master secret",
                                     &client_hello_rand,
@@ -757,8 +769,9 @@ pub fn init(input: *Reader, output: *Writer, options: Options) InitError!Client
                                     nonce,
                                     pv.app_cipher.client_write_key,
                                 );
-                                var all_msgs_vec: [3][]const u8 = .{
-                                    &client_key_exchange_msg,
+                                var all_msgs_vec: [4][]const u8 = .{
+                                    &client_key_exchange_prefix,
+                                    public_key_bytes,
                                     &client_change_cipher_spec_msg,
                                     &client_verify_msg,
                                 };