diff --git a/lib/std/crypto/chacha20.zig b/lib/std/crypto/chacha20.zig
index 10d2130659..8a0f677660 100644
--- a/lib/std/crypto/chacha20.zig
+++ b/lib/std/crypto/chacha20.zig
@@ -7,6 +7,7 @@ const assert = std.debug.assert;
 const testing = std.testing;
 const builtin = @import("builtin");
 const maxInt = std.math.maxInt;
+const Poly1305 = std.crypto.Poly1305;
 
 const QuarterRound = struct {
     a: usize,
@@ -434,3 +435,205 @@ test "crypto.chacha20 test vector 5" {
     chaCha20With64BitNonce(result[0..], input[0..], 0, key, nonce);
     testing.expectEqualSlices(u8, &expected_result, &result);
 }
+
+pub const chacha20poly1305_tag_size = 16;
+
+pub fn chacha20poly1305Seal(dst: []u8, plaintext: []const u8, data: []const u8, key: [32]u8, nonce: [12]u8) void {
+    assert(dst.len >= plaintext.len + chacha20poly1305_tag_size);
+
+    // derive poly1305 key
+    var polyKey = [_]u8{0} ** 32;
+    chaCha20IETF(polyKey[0..], polyKey[0..], 0, key, nonce);
+
+    // encrypt plaintext
+    chaCha20IETF(dst[0..plaintext.len], plaintext, 1, key, nonce);
+
+    // construct mac
+    var mac = Poly1305.init(polyKey[0..]);
+    mac.update(data);
+    if (data.len % 16 != 0) {
+        const zeros = [_]u8{0} ** 16;
+        const padding = 16 - (data.len % 16);
+        mac.update(zeros[0..padding]);
+    }
+    mac.update(dst[0..plaintext.len]);
+    if (plaintext.len % 16 != 0) {
+        const zeros = [_]u8{0} ** 16;
+        const padding = 16 - (plaintext.len % 16);
+        mac.update(zeros[0..padding]);
+    }
+    var lens: [16]u8 = undefined;
+    mem.writeIntSliceLittle(u64, lens[0..8], data.len);
+    mem.writeIntSliceLittle(u64, lens[8..16], plaintext.len);
+    mac.update(lens[0..]);
+    mac.final(dst[plaintext.len..]);
+}
+
+/// Verifies and decrypts an authenticated message produced by chacha20poly1305Seal.
+pub fn chacha20poly1305Open(dst: []u8, msgAndTag: []const u8, data: []const u8, key: [32]u8, nonce: [12]u8) !void {
+    if (msgAndTag.len < chacha20poly1305_tag_size) {
+        return error.InvalidMessage;
+    }
+
+    // split ciphertext and tag
+    assert(dst.len >= msgAndTag.len - chacha20poly1305_tag_size);
+    var ciphertext = msgAndTag[0 .. msgAndTag.len - chacha20poly1305_tag_size];
+    var polyTag = msgAndTag[ciphertext.len..];
+
+    // derive poly1305 key
+    var polyKey = [_]u8{0} ** 32;
+    chaCha20IETF(polyKey[0..], polyKey[0..], 0, key, nonce);
+
+    // construct mac
+    var mac = Poly1305.init(polyKey[0..]);
+
+    mac.update(data);
+    if (data.len % 16 != 0) {
+        const zeros = [_]u8{0} ** 16;
+        const padding = 16 - (data.len % 16);
+        mac.update(zeros[0..padding]);
+    }
+    mac.update(ciphertext);
+    if (ciphertext.len % 16 != 0) {
+        const zeros = [_]u8{0} ** 16;
+        const padding = 16 - (ciphertext.len % 16);
+        mac.update(zeros[0..padding]);
+    }
+    var lens: [16]u8 = undefined;
+    mem.writeIntSliceLittle(u64, lens[0..8], data.len);
+    mem.writeIntSliceLittle(u64, lens[8..16], ciphertext.len);
+    mac.update(lens[0..]);
+    var computedTag: [16]u8 = undefined;
+    mac.final(computedTag[0..]);
+
+    // verify mac in constant time
+    // TODO: we can't currently guarantee that this will run in constant time.
+    // See https://github.com/ziglang/zig/issues/1776
+    var acc: u8 = 0;
+    for (computedTag) |_, i| {
+        acc |= (computedTag[i] ^ polyTag[i]);
+    }
+    if (acc != 0) {
+        return error.AuthenticationFailed;
+    }
+
+    // decrypt ciphertext
+    chaCha20IETF(dst[0..ciphertext.len], ciphertext, 1, key, nonce);
+}
+
+test "seal" {
+    {
+        const plaintext = "";
+        const data = "";
+        const key = [_]u8{
+            0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
+            0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
+        };
+        const nonce = [_]u8{ 0x7, 0x0, 0x0, 0x0, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47 };
+        const exp_out = [_]u8{ 0xa0, 0x78, 0x4d, 0x7a, 0x47, 0x16, 0xf3, 0xfe, 0xb4, 0xf6, 0x4e, 0x7f, 0x4b, 0x39, 0xbf, 0x4 };
+
+        var out: [exp_out.len]u8 = undefined;
+        chacha20poly1305Seal(out[0..], plaintext, data, key, nonce);
+        testing.expectEqualSlices(u8, exp_out[0..], out[0..]);
+    }
+    {
+        const plaintext = [_]u8{
+            0x4c, 0x61, 0x64, 0x69, 0x65, 0x73, 0x20, 0x61, 0x6e, 0x64, 0x20, 0x47, 0x65, 0x6e, 0x74, 0x6c,
+            0x65, 0x6d, 0x65, 0x6e, 0x20, 0x6f, 0x66, 0x20, 0x74, 0x68, 0x65, 0x20, 0x63, 0x6c, 0x61, 0x73,
+            0x73, 0x20, 0x6f, 0x66, 0x20, 0x27, 0x39, 0x39, 0x3a, 0x20, 0x49, 0x66, 0x20, 0x49, 0x20, 0x63,
+            0x6f, 0x75, 0x6c, 0x64, 0x20, 0x6f, 0x66, 0x66, 0x65, 0x72, 0x20, 0x79, 0x6f, 0x75, 0x20, 0x6f,
+            0x6e, 0x6c, 0x79, 0x20, 0x6f, 0x6e, 0x65, 0x20, 0x74, 0x69, 0x70, 0x20, 0x66, 0x6f, 0x72, 0x20,
+            0x74, 0x68, 0x65, 0x20, 0x66, 0x75, 0x74, 0x75, 0x72, 0x65, 0x2c, 0x20, 0x73, 0x75, 0x6e, 0x73,
+            0x63, 0x72, 0x65, 0x65, 0x6e, 0x20, 0x77, 0x6f, 0x75, 0x6c, 0x64, 0x20, 0x62, 0x65, 0x20, 0x69,
+            0x74, 0x2e,
+        };
+        const data = [_]u8{ 0x50, 0x51, 0x52, 0x53, 0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7 };
+        const key = [_]u8{
+            0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
+            0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
+        };
+        const nonce = [_]u8{ 0x7, 0x0, 0x0, 0x0, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47 };
+        const exp_out = [_]u8{
+            0xd3, 0x1a, 0x8d, 0x34, 0x64, 0x8e, 0x60, 0xdb, 0x7b, 0x86, 0xaf, 0xbc, 0x53, 0xef, 0x7e, 0xc2,
+            0xa4, 0xad, 0xed, 0x51, 0x29, 0x6e, 0x8,  0xfe, 0xa9, 0xe2, 0xb5, 0xa7, 0x36, 0xee, 0x62, 0xd6,
+            0x3d, 0xbe, 0xa4, 0x5e, 0x8c, 0xa9, 0x67, 0x12, 0x82, 0xfa, 0xfb, 0x69, 0xda, 0x92, 0x72, 0x8b,
+            0x1a, 0x71, 0xde, 0xa,  0x9e, 0x6,  0xb,  0x29, 0x5,  0xd6, 0xa5, 0xb6, 0x7e, 0xcd, 0x3b, 0x36,
+            0x92, 0xdd, 0xbd, 0x7f, 0x2d, 0x77, 0x8b, 0x8c, 0x98, 0x3,  0xae, 0xe3, 0x28, 0x9,  0x1b, 0x58,
+            0xfa, 0xb3, 0x24, 0xe4, 0xfa, 0xd6, 0x75, 0x94, 0x55, 0x85, 0x80, 0x8b, 0x48, 0x31, 0xd7, 0xbc,
+            0x3f, 0xf4, 0xde, 0xf0, 0x8e, 0x4b, 0x7a, 0x9d, 0xe5, 0x76, 0xd2, 0x65, 0x86, 0xce, 0xc6, 0x4b,
+            0x61, 0x16, 0x1a, 0xe1, 0xb,  0x59, 0x4f, 0x9,  0xe2, 0x6a, 0x7e, 0x90, 0x2e, 0xcb, 0xd0, 0x60,
+            0x6,  0x91,
+        };
+
+        var out: [exp_out.len]u8 = undefined;
+        chacha20poly1305Seal(out[0..], plaintext[0..], data[0..], key, nonce);
+        testing.expectEqualSlices(u8, exp_out[0..], out[0..]);
+    }
+}
+
+test "open" {
+    {
+        const ciphertext = [_]u8{ 0xa0, 0x78, 0x4d, 0x7a, 0x47, 0x16, 0xf3, 0xfe, 0xb4, 0xf6, 0x4e, 0x7f, 0x4b, 0x39, 0xbf, 0x4 };
+        const data = "";
+        const key = [_]u8{
+            0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
+            0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
+        };
+        const nonce = [_]u8{ 0x7, 0x0, 0x0, 0x0, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47 };
+        const exp_out = "";
+
+        var out: [exp_out.len]u8 = undefined;
+        try chacha20poly1305Open(out[0..], ciphertext[0..], data, key, nonce);
+        testing.expectEqualSlices(u8, exp_out[0..], out[0..]);
+    }
+    {
+        const ciphertext = [_]u8{
+            0xd3, 0x1a, 0x8d, 0x34, 0x64, 0x8e, 0x60, 0xdb, 0x7b, 0x86, 0xaf, 0xbc, 0x53, 0xef, 0x7e, 0xc2,
+            0xa4, 0xad, 0xed, 0x51, 0x29, 0x6e, 0x8,  0xfe, 0xa9, 0xe2, 0xb5, 0xa7, 0x36, 0xee, 0x62, 0xd6,
+            0x3d, 0xbe, 0xa4, 0x5e, 0x8c, 0xa9, 0x67, 0x12, 0x82, 0xfa, 0xfb, 0x69, 0xda, 0x92, 0x72, 0x8b,
+            0x1a, 0x71, 0xde, 0xa,  0x9e, 0x6,  0xb,  0x29, 0x5,  0xd6, 0xa5, 0xb6, 0x7e, 0xcd, 0x3b, 0x36,
+            0x92, 0xdd, 0xbd, 0x7f, 0x2d, 0x77, 0x8b, 0x8c, 0x98, 0x3,  0xae, 0xe3, 0x28, 0x9,  0x1b, 0x58,
+            0xfa, 0xb3, 0x24, 0xe4, 0xfa, 0xd6, 0x75, 0x94, 0x55, 0x85, 0x80, 0x8b, 0x48, 0x31, 0xd7, 0xbc,
+            0x3f, 0xf4, 0xde, 0xf0, 0x8e, 0x4b, 0x7a, 0x9d, 0xe5, 0x76, 0xd2, 0x65, 0x86, 0xce, 0xc6, 0x4b,
+            0x61, 0x16, 0x1a, 0xe1, 0xb,  0x59, 0x4f, 0x9,  0xe2, 0x6a, 0x7e, 0x90, 0x2e, 0xcb, 0xd0, 0x60,
+            0x6,  0x91,
+        };
+        const data = [_]u8{ 0x50, 0x51, 0x52, 0x53, 0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7 };
+        const key = [_]u8{
+            0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
+            0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
+        };
+        const nonce = [_]u8{ 0x7, 0x0, 0x0, 0x0, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47 };
+        const exp_out = [_]u8{
+            0x4c, 0x61, 0x64, 0x69, 0x65, 0x73, 0x20, 0x61, 0x6e, 0x64, 0x20, 0x47, 0x65, 0x6e, 0x74, 0x6c,
+            0x65, 0x6d, 0x65, 0x6e, 0x20, 0x6f, 0x66, 0x20, 0x74, 0x68, 0x65, 0x20, 0x63, 0x6c, 0x61, 0x73,
+            0x73, 0x20, 0x6f, 0x66, 0x20, 0x27, 0x39, 0x39, 0x3a, 0x20, 0x49, 0x66, 0x20, 0x49, 0x20, 0x63,
+            0x6f, 0x75, 0x6c, 0x64, 0x20, 0x6f, 0x66, 0x66, 0x65, 0x72, 0x20, 0x79, 0x6f, 0x75, 0x20, 0x6f,
+            0x6e, 0x6c, 0x79, 0x20, 0x6f, 0x6e, 0x65, 0x20, 0x74, 0x69, 0x70, 0x20, 0x66, 0x6f, 0x72, 0x20,
+            0x74, 0x68, 0x65, 0x20, 0x66, 0x75, 0x74, 0x75, 0x72, 0x65, 0x2c, 0x20, 0x73, 0x75, 0x6e, 0x73,
+            0x63, 0x72, 0x65, 0x65, 0x6e, 0x20, 0x77, 0x6f, 0x75, 0x6c, 0x64, 0x20, 0x62, 0x65, 0x20, 0x69,
+            0x74, 0x2e,
+        };
+
+        var out: [exp_out.len]u8 = undefined;
+        try chacha20poly1305Open(out[0..], ciphertext[0..], data[0..], key, nonce);
+        testing.expectEqualSlices(u8, exp_out[0..], out[0..]);
+
+        // corrupting the ciphertext, data, key, or nonce should cause a failure
+        var bad_ciphertext = ciphertext;
+        bad_ciphertext[0] ^= 1;
+        testing.expectError(error.AuthenticationFailed, chacha20poly1305Open(out[0..], bad_ciphertext[0..], data[0..], key, nonce));
+        var bad_data = data;
+        bad_data[0] ^= 1;
+        testing.expectError(error.AuthenticationFailed, chacha20poly1305Open(out[0..], ciphertext[0..], bad_data[0..], key, nonce));
+        var bad_key = key;
+        bad_key[0] ^= 1;
+        testing.expectError(error.AuthenticationFailed, chacha20poly1305Open(out[0..], ciphertext[0..], data[0..], bad_key, nonce));
+        var bad_nonce = nonce;
+        bad_nonce[0] ^= 1;
+        testing.expectError(error.AuthenticationFailed, chacha20poly1305Open(out[0..], ciphertext[0..], data[0..], key, bad_nonce));
+
+        // a short ciphertext should result in a different error
+        testing.expectError(error.InvalidMessage, chacha20poly1305Open(out[0..], "", data[0..], key, bad_nonce));
+    }
+}