diff --git a/src/hmac.rs b/src/hmac.rs
new file mode 100644
index 0000000..601ae01
--- /dev/null
+++ b/src/hmac.rs
@@ -0,0 +1,68 @@
+use crate::cvt_p;
+use crate::error::ErrorStack;
+use crate::md::MdRef;
+use foreign_types::ForeignTypeRef;
+use openssl_macros::corresponds;
+use libc::{c_void, c_uint};
+use std::convert::TryFrom;
+
+/// Computes the HMAC as a one-shot operation.
+///
+/// Calculates the HMAC of data, using the given |key|
+/// and hash function |md|, and returns the result re-using the space from
+/// buffer |out|. On entry, |out| must contain at least |EVP_MD_size| bytes
+/// of space. The actual length of the result is used to resize the returned
+/// slice. An output size of |EVP_MAX_MD_SIZE| will always be large enough.
+/// It returns a resized |out| or ErrorStack on error.
+#[corresponds(HMAC)]
+#[inline]
+pub fn hmac<'a>(
+    md: &MdRef,
+    key: &[u8],
+    data: &[u8],
+    out: &'a mut [u8]
+) -> Result<&'a [u8], ErrorStack> {
+    let mut out_len = c_uint::try_from(out.len()).unwrap();
+    unsafe {
+        cvt_p(ffi::HMAC(
+            md.as_ptr(),
+            key.as_ptr() as *const c_void,
+            key.len(),
+            data.as_ptr(),
+            data.len(),
+            out.as_mut_ptr(),
+            &mut out_len
+            ))?;
+    }
+    Ok(&out[..out_len as usize])
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::md::Md;
+    use crate::memcmp;
+
+    const SHA_256_DIGEST_SIZE:usize = 32;
+
+    #[test]
+    fn hmac_sha256_test() {
+        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];
+        let mut out: [u8; SHA_256_DIGEST_SIZE] = [0; SHA_256_DIGEST_SIZE];
+        let key:[u8; 20] = [0x0b; 20];
+        let data = b"Hi There";
+        let hmac_result = hmac(Md::sha256(), &key, data, &mut out).expect("Couldn't calculate sha256 hmac");
+        expect!(memcmp::eq(&hmac_result, &expected_hmac));
+    }
+
+    #[test]
+    fn hmac_sha256_test_big_buffer() {
+        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];
+        let mut out: [u8; 100] = [0; 100];
+        let key:[u8;20] = [0x0b; 20];
+        let data = b"Hi There";
+        let hmac_result = hmac(Md::sha256(), &key, data, &mut out).expect("Couldn't calculate sha256 hmac");
+        expect_eq!(hmac_result.len(), SHA_256_DIGEST_SIZE);
+        expect!(memcmp::eq(&hmac_result, &expected_hmac));
+    }
+}