1diff --git a/src/hmac.rs b/src/hmac.rs
2new file mode 100644
3index 0000000..601ae01
4--- /dev/null
5+++ b/src/hmac.rs
6@@ -0,0 +1,68 @@
7+use crate::cvt_p;
8+use crate::error::ErrorStack;
9+use crate::md::MdRef;
10+use foreign_types::ForeignTypeRef;
11+use openssl_macros::corresponds;
12+use libc::{c_void, c_uint};
13+use std::convert::TryFrom;
14+
15+/// Computes the HMAC as a one-shot operation.
16+///
17+/// Calculates the HMAC of data, using the given |key|
18+/// and hash function |md|, and returns the result re-using the space from
19+/// buffer |out|. On entry, |out| must contain at least |EVP_MD_size| bytes
20+/// of space. The actual length of the result is used to resize the returned
21+/// slice. An output size of |EVP_MAX_MD_SIZE| will always be large enough.
22+/// It returns a resized |out| or ErrorStack on error.
23+#[corresponds(HMAC)]
24+#[inline]
25+pub fn hmac<'a>(
26+    md: &MdRef,
27+    key: &[u8],
28+    data: &[u8],
29+    out: &'a mut [u8]
30+) -> Result<&'a [u8], ErrorStack> {
31+    let mut out_len = c_uint::try_from(out.len()).unwrap();
32+    unsafe {
33+        cvt_p(ffi::HMAC(
34+            md.as_ptr(),
35+            key.as_ptr() as *const c_void,
36+            key.len(),
37+            data.as_ptr(),
38+            data.len(),
39+            out.as_mut_ptr(),
40+            &mut out_len
41+            ))?;
42+    }
43+    Ok(&out[..out_len as usize])
44+}
45+
46+#[cfg(test)]
47+mod tests {
48+    use super::*;
49+    use crate::md::Md;
50+    use crate::memcmp;
51+
52+    const SHA_256_DIGEST_SIZE:usize = 32;
53+
54+    #[test]
55+    fn hmac_sha256_test() {
56+        let expected_hmac = [0xb0, 0x34, 0x4c, 0x61, 0xd8, 0xdb, 0x38, 0x53, 0x5c, 0xa8, 0xaf, 0xce, 0xaf, 0xb, 0xf1, 0x2b, 0x88, 0x1d, 0xc2, 0x0, 0xc9, 0x83, 0x3d, 0xa7, 0x26, 0xe9, 0x37, 0x6c, 0x2e, 0x32, 0xcf, 0xf7];
57+        let mut out: [u8; SHA_256_DIGEST_SIZE] = [0; SHA_256_DIGEST_SIZE];
58+        let key:[u8; 20] = [0x0b; 20];
59+        let data = b"Hi There";
60+        let hmac_result = hmac(Md::sha256(), &key, data, &mut out).expect("Couldn't calculate sha256 hmac");
61+        expect!(memcmp::eq(&hmac_result, &expected_hmac));
62+    }
63+
64+    #[test]
65+    fn hmac_sha256_test_big_buffer() {
66+        let expected_hmac = [0xb0, 0x34, 0x4c, 0x61, 0xd8, 0xdb, 0x38, 0x53, 0x5c, 0xa8, 0xaf, 0xce, 0xaf, 0xb, 0xf1, 0x2b, 0x88, 0x1d, 0xc2, 0x0, 0xc9, 0x83, 0x3d, 0xa7, 0x26, 0xe9, 0x37, 0x6c, 0x2e, 0x32, 0xcf, 0xf7];
67+        let mut out: [u8; 100] = [0; 100];
68+        let key:[u8;20] = [0x0b; 20];
69+        let data = b"Hi There";
70+        let hmac_result = hmac(Md::sha256(), &key, data, &mut out).expect("Couldn't calculate sha256 hmac");
71+        expect_eq!(hmac_result.len(), SHA_256_DIGEST_SIZE);
72+        expect!(memcmp::eq(&hmac_result, &expected_hmac));
73+    }
74+}
75