diff --git a/securesystemslib/_internal/utils.py b/securesystemslib/_internal/utils.py
new file mode 100644
index 00000000..70bfba93
--- /dev/null
+++ b/securesystemslib/_internal/utils.py
@@ -0,0 +1,41 @@
+"""Internal utilities"""
+
+import base64
+import binascii
+
+
+def b64enc(data: bytes) -> str:
+    """To encode byte sequence into base64 string
+
+    Arguments:
+        data: Byte sequence to encode
+
+    Exceptions:
+        TypeError: If "data" is not byte sequence
+
+    Returns:
+        base64 string
+    """
+
+    return base64.standard_b64encode(data).decode("utf-8")
+
+
+def b64dec(string: str) -> bytes:
+    """To decode byte sequence from base64 string
+
+    Arguments:
+        string: base64 string to decode
+
+    Raises:
+        binascii.Error: If invalid base64-encoded string
+
+    Returns:
+        A byte sequence
+    """
+
+    data = string.encode("utf-8")
+    try:
+        return base64.b64decode(data, validate=True)
+    except binascii.Error:
+        # altchars for urlsafe encoded base64 - instead of + and _ instead of /
+        return base64.b64decode(data, altchars=b"-_", validate=True)
diff --git a/securesystemslib/dsse.py b/securesystemslib/dsse.py
new file mode 100644
index 00000000..c1e41a28
--- /dev/null
+++ b/securesystemslib/dsse.py
@@ -0,0 +1,162 @@
+"""Dead Simple Signing Envelope
+"""
+
+import logging
+from typing import Any, Dict, List
+
+from securesystemslib import exceptions
+from securesystemslib._internal.utils import b64dec, b64enc
+from securesystemslib.signer import Key, Signature, Signer
+
+logger = logging.getLogger(__name__)
+
+
+class Envelope:
+    """DSSE Envelope to provide interface for signing arbitrary data.
+
+    Attributes:
+        payload: Arbitrary byte sequence of serialized body.
+        payload_type: string that identifies how to interpret payload.
+        signatures: list of Signature.
+
+    """
+
+    def __init__(
+        self, payload: bytes, payload_type: str, signatures: List[Signature]
+    ):
+        self.payload = payload
+        self.payload_type = payload_type
+        self.signatures = signatures
+
+    def __eq__(self, other: Any) -> bool:
+        if not isinstance(other, Envelope):
+            return False
+
+        return (
+            self.payload == other.payload
+            and self.payload_type == other.payload_type
+            and self.signatures == other.signatures
+        )
+
+    @classmethod
+    def from_dict(cls, data: dict) -> "Envelope":
+        """Creates a DSSE Envelope from its JSON/dict representation.
+
+        Arguments:
+            data: A dict containing a valid payload, payloadType and signatures
+
+        Raises:
+            KeyError: If any of the "payload", "payloadType" and "signatures"
+                fields are missing from the "data".
+
+            FormatError: If signature in "signatures" is incorrect.
+
+        Returns:
+            A "Envelope" instance.
+        """
+
+        payload = b64dec(data["payload"])
+        payload_type = data["payloadType"]
+
+        signatures = [
+            Signature.from_dict(signature) for signature in data["signatures"]
+        ]
+
+        return cls(payload, payload_type, signatures)
+
+    def to_dict(self) -> dict:
+        """Returns the JSON-serializable dictionary representation of self."""
+
+        return {
+            "payload": b64enc(self.payload),
+            "payloadType": self.payload_type,
+            "signatures": [
+                signature.to_dict() for signature in self.signatures
+            ],
+        }
+
+    def pae(self) -> bytes:
+        """Pre-Auth-Encoding byte sequence of self."""
+
+        return b"DSSEv1 %d %b %d %b" % (
+            len(self.payload_type),
+            self.payload_type.encode("utf-8"),
+            len(self.payload),
+            self.payload,
+        )
+
+    def sign(self, signer: Signer) -> Signature:
+        """Sign the payload and create the signature.
+
+        Arguments:
+            signer: A "Signer" class instance.
+
+        Returns:
+            A "Signature" instance.
+        """
+
+        signature = signer.sign(self.pae())
+        self.signatures.append(signature)
+
+        return signature
+
+    def verify(self, keys: List[Key], threshold: int) -> Dict[str, Key]:
+        """Verify the payload with the provided Keys.
+
+        NOTE: This API is experimental and might change (see
+        secure-systems-lab/dsse#55)
+
+        Arguments:
+            keys: A list of public keys to verify the signatures.
+            threshold: Number of signatures needed to pass the verification.
+
+        Raises:
+            ValueError: If "threshold" is not valid.
+            VerificationError: If the enclosed signatures do not pass the
+                verification.
+
+        Note:
+            Mandating keyid in signatures and matching them with keyid of Key
+            in order to consider them for verification, is not DSSE spec
+            compliant (Issue #416).
+
+        Returns:
+            A dict of the threshold of unique public keys that verified a
+            signature.
+        """
+
+        accepted_keys = {}
+        pae = self.pae()
+
+        # checks for threshold value.
+        if threshold <= 0:
+            raise ValueError("Threshold must be greater than 0")
+
+        if len(keys) < threshold:
+            raise ValueError("Number of keys can't be less than threshold")
+
+        for signature in self.signatures:
+            for key in keys:
+                # If Signature keyid doesn't match with Key, skip.
+                if not key.keyid == signature.keyid:
+                    continue
+
+                # If a key verifies the signature, we exit and use the result.
+                try:
+                    key.verify_signature(signature, pae)
+                    accepted_keys[key.keyid] = key
+                    break
+                except exceptions.UnverifiedSignatureError:
+                    continue
+
+            # Break, if amount of accepted_keys are more than threshold.
+            if len(accepted_keys) >= threshold:
+                break
+
+        if threshold > len(accepted_keys):
+            raise exceptions.VerificationError(
+                "Accepted signatures do not match threshold,"
+                f" Found: {len(accepted_keys)}, Expected {threshold}"
+            )
+
+        return accepted_keys
diff --git a/securesystemslib/exceptions.py b/securesystemslib/exceptions.py
index 5f25fbe0..3e8122a1 100755
--- a/securesystemslib/exceptions.py
+++ b/securesystemslib/exceptions.py
@@ -147,3 +147,11 @@ class UnverifiedSignatureError(Error):
 
 class VerificationError(UnverifiedSignatureError):
     """Signature could not be verified because something failed in the process"""
+
+
+class SerializationError(Error):
+    """Error during serialization."""
+
+
+class DeserializationError(Error):
+    """Error during deserialization."""
diff --git a/tests/test_dsse.py b/tests/test_dsse.py
new file mode 100644
index 00000000..a91893b4
--- /dev/null
+++ b/tests/test_dsse.py
@@ -0,0 +1,157 @@
+#!/usr/bin/env python
+
+"""Test cases for "metadata.py". """
+
+import copy
+import unittest
+
+import securesystemslib.keys as KEYS
+from securesystemslib.dsse import Envelope
+from securesystemslib.exceptions import (
+    FormatError,
+    UnsupportedAlgorithmError,
+    VerificationError,
+)
+from securesystemslib.signer import Signature, SSlibKey, SSlibSigner
+
+
+class TestEnvelope(unittest.TestCase):
+    """Test metadata interface provided by DSSE envelope."""
+
+    @classmethod
+    def setUpClass(cls):
+        cls.key_dicts = [
+            KEYS.generate_rsa_key(),
+            KEYS.generate_ed25519_key(),
+            KEYS.generate_ecdsa_key(),
+        ]
+
+        cls.signature_dict = {
+            "keyid": "11fa391a0ed7a447",
+            "sig": "30460221009342e4566528fcecf6a7a5",
+        }
+        cls.envelope_dict = {
+            "payload": "aGVsbG8gd29ybGQ=",
+            "payloadType": "http://example.com/HelloWorld",
+            "signatures": [cls.signature_dict],
+        }
+        cls.pae = b"DSSEv1 29 http://example.com/HelloWorld 11 hello world"
+
+    def test_envelope_from_to_dict(self):
+        """Test envelope to_dict and from_dict methods."""
+
+        envelope_dict = copy.deepcopy(self.envelope_dict)
+
+        # create envelope object from its dict.
+        envelope_obj = Envelope.from_dict(envelope_dict)
+        for signature in envelope_obj.signatures:
+            self.assertIsInstance(signature, Signature)
+
+        # Assert envelope dict created by to_dict will be equal.
+        self.assertDictEqual(self.envelope_dict, envelope_obj.to_dict())
+
+    def test_envelope_eq_(self):
+        """Test envelope equality."""
+
+        envelope_obj = Envelope.from_dict(copy.deepcopy(self.envelope_dict))
+
+        # Assert that object and None will not be equal.
+        self.assertNotEqual(None, envelope_obj)
+
+        # Assert a copy of envelope_obj will be equal to envelope_obj.
+        envelope_obj_2 = copy.deepcopy(envelope_obj)
+        self.assertEqual(envelope_obj, envelope_obj_2)
+
+        # Assert that changing the "payload" will make the objects not equal.
+        envelope_obj_2.payload = b"wrong_payload"
+        self.assertNotEqual(envelope_obj, envelope_obj_2)
+        envelope_obj_2.payload = envelope_obj.payload
+
+        # Assert that changing the "payload_type" will make the objects not equal.
+        envelope_obj_2.payload_type = "wrong_payload_type"
+        self.assertNotEqual(envelope_obj, envelope_obj_2)
+        envelope_obj_2.payload = envelope_obj.payload
+
+        # Assert that changing the "signatures" will make the objects not equal.
+        sig_obg = Signature("", self.signature_dict["sig"])
+        envelope_obj_2.signatures = [sig_obg]
+        self.assertNotEqual(envelope_obj, envelope_obj_2)
+
+    def test_preauthencoding(self):
+        """Test envelope Pre-Auth-Encoding."""
+
+        envelope_obj = Envelope.from_dict(copy.deepcopy(self.envelope_dict))
+
+        # Checking for Pre-Auth-Encoding generated is correct.
+        self.assertEqual(self.pae, envelope_obj.pae())
+
+    def test_sign_and_verify(self):
+        """Test for creating and verifying DSSE signatures."""
+
+        # Create an Envelope with no signatures.
+        envelope_dict = copy.deepcopy(self.envelope_dict)
+        envelope_dict["signatures"] = []
+        envelope_obj = Envelope.from_dict(envelope_dict)
+
+        key_list = []
+        for key_dict in self.key_dicts:
+            # Test for invalid scheme.
+            valid_scheme = key_dict["scheme"]
+            key_dict["scheme"] = "invalid_scheme"
+            signer = SSlibSigner(key_dict)
+            with self.assertRaises((FormatError, UnsupportedAlgorithmError)):
+                envelope_obj.sign(signer)
+
+            # Sign the payload.
+            key_dict["scheme"] = valid_scheme
+            signer = SSlibSigner(key_dict)
+            envelope_obj.sign(signer)
+
+            # Create a List of "Key" from key_dict.
+            key_list.append(SSlibKey.from_securesystemslib_key(key_dict))
+
+        # Check for signatures of Envelope.
+        self.assertEqual(len(self.key_dicts), len(envelope_obj.signatures))
+        for signature in envelope_obj.signatures:
+            self.assertIsInstance(signature, Signature)
+
+        # Test for invalid threshold value for keys_list.
+        # threshold is 0.
+        with self.assertRaises(ValueError):
+            envelope_obj.verify(key_list, 0)
+
+        # threshold is greater than no of keys.
+        with self.assertRaises(ValueError):
+            envelope_obj.verify(key_list, 4)
+
+        # Test with valid keylist and threshold.
+        verified_keys = envelope_obj.verify(key_list, len(key_list))
+        self.assertEqual(len(verified_keys), len(key_list))
+
+        # Test for unknown keys and threshold of 1.
+        new_key_dicts = [
+            KEYS.generate_rsa_key(),
+            KEYS.generate_ed25519_key(),
+            KEYS.generate_ecdsa_key(),
+        ]
+        new_key_list = []
+        for key_dict in new_key_dicts:
+            new_key_list.append(SSlibKey.from_securesystemslib_key(key_dict))
+
+        with self.assertRaises(VerificationError):
+            envelope_obj.verify(new_key_list, 1)
+
+        all_keys = key_list + new_key_list
+        envelope_obj.verify(all_keys, 3)
+
+        # Test with duplicate keys.
+        duplicate_keys = key_list + key_list
+        with self.assertRaises(VerificationError):
+            envelope_obj.verify(
+                duplicate_keys, 4
+            )  # 3 unique keys, threshold 4.
+
+
+# Run the unit tests.
+if __name__ == "__main__":
+    unittest.main()